CDO Genetic and Medical Glossary

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46,XY complete gonadal dysgenesis: disorder of sex development (DSD) associated with anomalies in gonadal development that result in the presence of female external and internal genitalia despite the 46,XY karyotype

A: the abbreviation for adenine, a purine nitrogenous base found in the structure of both DNA and RNA
Aberration: characteristic that deviates from the norm
Ablepharon: absent eyelids
Acardiac twin: also known as twin reversed-arterial perfusion (TRAP) sequence; rare and serious pregnancy complication in which blood is delivered from one twin (“pump” twin) to the other twin (“acardiac” twin) by retrograde (backward) flow, meaning that blood is being pumped from the pump twin to the acardiac twin which is depleted of oxygen
Acentric chromosome: a chromosome that does not have a centromere and that is unable to participate properly in cell division; often the result of a chromosomal mutation during recombination
Acrocentric chromosome: a chromosome with its centromere near one end; includes chromosomes 13, 14, 15, 21 and 22 which all have their centromere very close to the outer tip; they have very short p arms and small round appendages on stalks, called ‘satellites’
Acrocephaly: turricephaly (tall head) with the top of the skull assuming a cone shape
Acro–renal–ocular syndrome: characterized by the association of acral, ocular and renal defects; acral anomalies are defects of the radial ray varying from mild hypoplasia of the distal part of the thumb with limitation of motion at the interphalangeal joint to severe thumb hypoplasia and preaxial polydactyly (thumb duplication), sometimes associated with syndactyly of fingers; renal anomalies vary from mild malrotation to crossed renal ectopia with partial horseshoe anomaly; ocular features include coloboma of the optic nerve, ptosis, and/or Duane anomaly
Adactyly: absence of all phalanges of all digits of a limb and associated soft tissues
Agangliotic megacolon: Hirschsprung’s disease
Agenesis: absence of
Agenesis of the corpus callosum: partial or complete absence of the area of the brain that connects the two cerebral hemispheres; can be inherited as an autosomal recessive trait, an X-linked dominant trait, or be caused by an infection or injury during the twelfth to twenty-second week of pregnancy
Ala nasi, cleft: notch in the margin of the ala nasi of the nose
Alacrimia: congenital inability to produce tears
Alagille syndrome: rare genetic disorder that can affect multiple organ systems of the body including the liver, heart, skeleton, eyes and kidneys; common symptoms usually develop during the first three months of life and include blockage of the flow of bile from the liver (cholestasis), yellowing of the skin and mucous membranes (jaundice), poor weight gain and growth, severe itching (pruritis) and pale, loose stools
Albright hereditary osteodystrophy: genetic condition characterized by short stature, bone and tooth abnormalities and cognitive/behavioral problems
Alfi syndrome: •9p Deletion Syndrome, usually where the breakpoint is found at 9p22; patients usually have various craniofacial abnormalities, globalized hypotonia (low muscle tone)/delayed gross motor development, seizures, GERD, constipation, cardiac defects, scoliosis, kidney reflux and/or sleep disorders
Allele: a form of a gene at a locus (site); each locus in an individual’s chromosomes has two alleles, which may be the same or different
Alobar HPE: complete lack of separation between the right and left halves of the forebrain
Alphafetoprotein (AFP): protein made by the fetus and found in the mother’s blood circulation; high level of AFP, either in the mother’s blood or in amniotic fluid may be associated with the presence of neural tube defects or chromosomal problems in the baby
Alpha-globin gene (a–globin gene): part of the gene cluster located on chromosome 16 spanning about 30 kb; deletion of both genes causes alpha thalassemias
Alpha–thalassemia (a–thalassemia): blood disorder that reduces the production of hemoglobin, the protein in red blood cells that carries oxygen to cells throughout the body; common symptoms include mild to moderate anemia, hepatosplenomegaly, and yellowing of the eyes and skin (jaundice)
Alveolar capillary dysplasia: life-threatening disorder of newborns in which the normal diffusion process of oxygen from the air sacs to the blood in the lungs (and then to the heart) fails to develop properly; pulmonary vein is malpositioned in a site somewhat different from its normal position.
Ambiguous genitalia: external sexual organs that are not clearly male or female
Amniocentesis: a procedure in which a small amount of amniotic fluid containing fetal cells is withdrawn from the amniotic sac surrounding a fetus; fetal cells, found in the fluid, are then tested for the presence of genetic abnormalities
Amniotic bands: constricted strands of the fluid-filled sac (amniotic sac) that surrounds a baby in the womb that can cause a congenital deformity of the face, arms, legs, fingers, or toes
Anal atresia: congenital absence of an opening at the bottom end of the intestinal tract; also called imperforate anus or anorectal atresia
Anencephalus (anencephaly): neural tube defect characterized by the failure of the cerebral hemispheres of the brain and the cranium to develop normally
Aneuploid: a cell or individual with one or more missing or extra chromosomes
Angelman syndrome: chromosomal disorder that is inherited from the mother (imprinting) and caused by deletion or abnormal expression of the UBE3A gene on chromosome 15; characterized by severe developmental delays and learning disabilities; absence or near absence of speech; inability to coordinate voluntary movements (ataxia); tremulous jerky movements of the arms and legs; patients have a characteristic happy disposition with unprovoked episodes of laughter and smiling—hence, the term “happy puppet syndrome”
Ankyloblepharon: partial fusion of the upper and lower eyelid margins by single or multiple bands of tissue
Ankyloglossia (tongue tie): short or anteriorly attached lingual frenulum associated with limited mobility of the tongue
Ankylosis: abnormal stiffening and immobility of a joint due to fusion of the bones
Annular pancreas: abnormal ring of pancreas that encircles the duodenum and often causes intestinal obstruction
Anodontia: congenital absence of all primary or permanent teeth
Anomaly: marked deviation from normal, especially as a result of congenital or hereditary defects
Anophthalmia: absence of the eye, as a result of a congenital malformation
Anosmia: loss of the sense of smell, either total or partial
Anotia: complete absence of any auricular structures, absent ear
Anterio–posterior: front to back direction
Anterior: frontal
Antibody: an immune protein (immunoglobulin) that is produced by B cells of the immune system and specifically recognizes an antigen
Anticipation: genetic condition that becomes more severe and/or arises at an earlier age as it is passed through subsequent generations (seen in some trinucleotide repeat mutations)
Anti-epileptic drug: medication to control seizures
Antigen: any molecule that is capable of being recognized by an antibody molecule or of provoking an immune response
Antisense: a term referring to any strand of DNA or RNA that is complementary to a coding or regulatory sequence
Aplasia: condition characterized by either partial or total loss of the ability to communicate verbally or to use or comprehend words
Aplastic: underdeveloped; defective development or congenital absence of a limb, organ
Apodia: absent foot
Aprosencephaly: congenital absence of the telencephalon and diencephalon parts of the forebrain
Arachnodactyly: long spider-like fingers and toes, a frequent finding in Marfan syndrome
Arachnoid cyst: benign cysts that occur in the cerebrospinal axis in relation to the arachnoid membrane and that do not communicate with the ventricular system
Arhinencephaly: congenital absence of the olfactory bulbs, tract or nerves
Array comparative genomic hybridization (CGH array): type of technology that allows detection of chromosome imbalances that too small to be detected through a microscope; works by exploiting the ability of a DNA molecule (or strand) to bind specifically to, or hybridize to, another DNA molecule (strand).
Arthrogryposis: joint contractures that develop before and are evident at birth
Ataxia: the loss of full muscle control of bodily movements
ATR–16 syndrome: 16p11.2 deletion syndrome; characterized by developmental delay, intellectual disability, and usually features of autism spectrum disorders
Atresia: absence or abnormal narrowing of an opening or passage in the body
Atretic: of, relating to, or marked by atresia
Atrial septal defect (ASD): ): congenital (present at birth) disorder of the heart in which infants have a hole in the wall (septum) that separates the two upper-chambers (atria) of the heart
Atrio-ventricular septal defect (AVSD): congenital (present at birth) disorder of the heart in which infants have improperly developed atrial and ventricular septa and adjoining valves
Atrophy: wasting away or decrease in size of an organ or tissue in the body
Attention deficit hyperactivity disorder (ADHD): any of a range of behavioral disorders occurring primarily in children, including such symptoms as poor concentration, hyperactivity, and impulsivity
Autism: developmental disorder that appears by age three and that is variable in expression but is recognized and diagnosed by impairment of the ability to form normal social relationships, by impairment of the ability to communicate with others, and by stereotyped behavior patterns especially as exhibited by a preoccupation with repetitive activities of restricted focus rather than with flexible and imaginative ones
Autoimmune response: an immune response of an organism against its own cells
Autonomic: part of the nervous system that regulates key involuntary functions of the body, including the activity of the heart muscle; the smooth muscles, including the muscles of the intestinal tract; and the glands
Autosomes: the numerical chromosomes (chromosomes 1-22); humans have forty-four —one set from each parent; chromosome 1 is the largest and 22, the smallest
Axenfeld–Rieger anomaly: eye disorder characterized by abnormalities of the front part of the eye (anterior segment) in which the colored part of the eye (the iris) may be thin or poorly developed
Axillary pterigyum: wing–like triangular membrane between the chin and neck

Balanced translocation: rearrangement of the chromosomes with no apparent loss or gain of chromosomal material; hence, a person with this rearrangement is not affected in any way; reciprocal translocation
Bannayan–Riley–Ruvalcaba syndrome: genetic condition characterized by a large head size (macrocephaly), multiple noncancerous tumors and tumor-like growths called hamartomas, and dark freckles on the penis in males; many have intellectual disability or delayed development, particularly the development of speech and of motor skills
Base pair (bp): measure of the size of a DNA fragment or the distance along a DNA molecule between markers
Bechçet syndrome: disorder that causes inflammation in blood vessels throughout the body, leading to mouth sores, eye inflammation, skin rashes and lesions, and genital sores; may have an autoimmune basis
Beckwith-Wiedemann syndrome: overgrowth disorder caused by paternal duplications (imprinting) of 11p15.5 on chromosome 11; characterized by above-average birth weight and increased growth after birth, an usually large tongue (macroglossia), enlargement of certain internal organs (visceromegaly), and protrusion of a portion of the intestines and abdominal organs through a tear in the wall of the stomach or bellybutton (abdominal wall defects)
Bicornis: refers to a uterus that is divided into two parts, usually separate at the upper end and joined at the lower end
Bicuspid aortic valve: aortic valve in the heart that has two flaps (cusps) that open and close, instead of the three flaps of a normal heart
Bifid uterus: an abnormal uterus that may be either a single or a double organ with two horns, or branches
Bifid uvula: bifurcation of the uvula, an incomplete form of cleft palate.
Biliary atresia: condition in which the bile ducts inside or outside the liver do not develop normally, causing bile that normally flows from the liver to the gallbladder to be blocked, which can lead to liver damage and cirrhosis
Bipolar disorder: a mental disorder marked by alternating periods of elation and depression
Blackfan–Diamond syndrome: blood disorder, characterized by a failure of the bone marrow (the center of the bone where blood cells are made) to produce red blood cells
Blaschko’s lines: pattern of distribution of skin lesions or pigmentary anomalies, linear on the extremities, S-shaped curves on the abdomen, and V-shaped on the back; thought to result from genetic mosaicism and interplay of transverse clonal proliferation and longitudinal growth and flexion of the embryo
Blepharochalasis: lax, wrinkled, and baggy eyelid skin
Blepharophimosis: fixed reduction in the vertical distance between the upper and lower eyelids with short palpebral fissures; often associated with epicanthus inversus
Brachycephaly: cephalic index (ratio of head width expressed as a percentage of head length) greater than 81%; shortened antero-posterior dimension (length) of the head compared to width
Brachydactyly of the hand: angulation of a digit toward the anterior axis (radial
Brachytelephalangy: short distance from the end of the finger to the most distal interphalangeal crease or DIPJ flexion point (ie, short fingers)
Brainstem: central trunk of the brain, consisting of the medulla oblongata, pons, and midbrain, and continuing downward to form the spinal cord
Branchial fistula: congenital epithelial cyst that arises on the lateral part of the neck due to failure of obliteration of the second branchial cleft in embryonic development
Branchio–oculo–facial syndrome: autosomal dominant genetic disorder characterized by the proliferation of blood vessels (hemangiomatous) in the lower neck or upper chest, low birth weight, retarded growth and degrees of intellectual impairment; patients may have a pseudo cleft of the upper lip resembling a poorly repaired cleft lip, a malformed nose with a broad bridge and flattened tip, blockage of the tear ducts, malformed ears, lumps in the area of the neck or collarbone and/or linear skin lesions behind the ears
Buschke–Ollendorf syndrome: hereditary disorder of connective tissue characterized by benign, connective tissue nevi (hamartomas) in the skin, and the formation of small, round spots of increased bone density (osteopoikilosis)

C: the abbreviation for cytosine, a pyrimidine nitrogenous base found in the structure of both DNA and RNA.
Café–au–lait spots: any of the medium brown spots usually found on the trunk, pelvis, and creases of the elbow
Campomelic dysplasia: syndrome featuring angulation of the long bones such as femur and tibiae
Camptodactyly: inability of the DIP joints and/or PIP joints of the fingers to be extended 180° by either active or passive means
Candidate gene: a gene suspected of causing a specific disease
Cardiomyopathy: chronic disease of the heart muscle
Carrier screening: specialized testing to see whether an individual carries a mutated gene for a specific condition
Carrier testing: testing of an individual who is at risk due to a family history to determine if he or she is a carrier of a mutated or faulty gene for a particular genetic condition
Carrier: a healthy individual who has one normal allele and one defective allele for a recessive genetic disease
Cat eye syndrome (tetrasomy 22cen-q11): chromosomal disorder in which the short arm and a small region of the long arm of chromosome 22 are duplicated four times; characterized by distinctive eye (ocular) abnormality that is sometimes present whereby there is partial absence of ocular tissue (coloboma), often affecting both eyes (bilateral); other symptoms include mild growth delays before birth, mild intellectual impairment, malformations of the skull and facial (craniofacial) region, the heart, the kidneys, and/or anal region; down-slanting eyelid folds (palpebral fissures), widely spaced eyes (ocular hypertelorism), misshapen ears with abnormal outgrowths of skin and small depressions in front of the outer ears (preauricular tags and pits), and/or absence (atresia) of the anal canal, with an abnormal passage (fistula) from the end portion of the large intestine (rectum) into abnormal locations
Caudal: at or near the tail or the posterior part of the body
Cebocephaly: obsolete, nonspecific term for a craniofacial malformation that has morphologic features of a monkey
Centromere: constricted part (central region of the chromosome that separates it into its two arms (the p arm and q arm)
Cerebellar vermis: structure of the brain located in the medial, cortico-nuclear zone of the cerebellum
Cerebellum: part of the brain at the back of the skull whose function is to coordinate and regulate muscular activity
Cerebral cortex: surface layer of gray tissue of the cerebrum, frequently called the gray matter
Cerebral dysgenesis: abnormal brain development in which the brain did not fully develop, grew abnormally, experienced incomplete division, or developed with incomplete organization
Charcot-Marie-Tooth disease type 1A: slowly progressive demyelinating peripheral neuropathy characterized by distal muscle weakness and atrophy, sensory loss, and slow nerve conduction velocity; often associated with pes cavus foot deformity and bilateral foot drop; caused by duplication of PMP22 located on chromosome 17 at 17p12
CHARGE syndrome: multisystem disorder caused by mutation or deletion of the CHD7 gene located at 8q12 on chromosome 8 and characterized by the presence of colobomas and cranial nerve defects (C) heart defects, especially tetralogy of Fallot (H), atresia of the choanae (blocked nasal breathing passages (A), retardation of growth and development (R), genital underdevelopment (G), and ear abnormalities and sensori-neural hearing loss (E)
Cheekbone underdevelopment: reduction in size of the zygomatic process of the temporal bone of the skull, which forms the middle and lateral inferior orbital margin; flat cheekbone, zygomatic underdevelopment
Chiari I malformation: occurs when part of the cerebellum is located below the foramen magnum; may develop when the bony space is smaller than normal, causing the cerebellum and brain stem to be pushed downward into the foramen magnum and into the upper spinal canal
Choanal atresia: narrowing or blockage of the nasal airway by tissue
Chondrodysplasia punctata: disorder of cartilage and bone development that occurs almost exclusively in males; abnormality that appears on x-rays as spots (stippling) near the ends of bones and in cartilage)
Chorionic villus sampling: a procedure in which fetal cells are obtained from an embryonic structure called the chorion and analyzed for the presence of genetic abnormalities in the fetus
Choroid: a thin vascular layer between the sclera and the retina of the eye; supplies blood to the retina and conducts arteries and nerves to other eye structures
Chorio-retinopathy: noninflammatory abnormality of the choroid with extension to the retina.
Chromatid: either of two threadlike strands into which a chromosome divides longitudinally during cell division; each contains a double helix of DNA
Chromatin: the material of which the chromosomes of organisms other than bacteria (i.e., eukaryotes) are composed. It consists of protein, RNA, and DNA
Chromosomal rearrangement: type of chromosome abnormality involving a change in the structure of the native chromosome and encompass several different classes of events: deletions, duplications, inversions; and translocations
Chromosome breakpoints: locations on a chromosome where DNA might get deleted, inverted, or swapped around
Chromosome map: a diagram showing the locations of genes on a particular chromosome; generated through analysis of linkage experiments involving those genes
Chromosome mutation: a change in chromosome structure caused by chromosome breakage followed by improper rejoining (eg, deletions, insertions, inversions, and translocations)
Chromosome recombination: process or act of exchanges of genes between chromosomes, resulting in a different genetic combination and ultimately to the formation of unique gametes with chromosomes that are different from those in parents
Chromosome: the form in which genetic material is found in the nucleus of a cell; composed of a single DNA molecule that is extremely tightly coiled and usually visible only during the processes of mitosis and meiosis
Cleft lip: fissure in the upper lip that is due to failure of the left and right sides of the fetal lip tissue to fuse; correctable by surgery
Cleido–cranial dysplasia: condition that primarily affects the development of the bones and teeth, resulting in underdeveloped or absent collarbones, delayed closing of the spaces between the bones of the skull, decreased bone density, delayed loss of the primary and delayed appearance of the secondary teeth, unusually shaped, peg-like teeth and misalignment of the teeth and jaws (malocclusion)
Clinical trial: an experimental research study used to determine the safety and effectiveness of a medical treatment or drug
Clone: a molecule, cell, or organism that is an exact genetic copy of another
Cloning vector: a DNA molecule that can be used to transport genes of interest into cells, where these genes can then be copied
Cloverleaf skull: trilobar skull configuration when viewed from the front or behind
Clubbing: broadening of the soft tissues of the digital tips in all dimensions associated with an increased longitudinal and lateral curvature of the nails
Club foot: congenital deformity of the ankles and/or feet in which the foot is turned inward and downward (talipes equinovarus), is angled at the heel with the toes pointing upward and outward (calcaneal valgus), or the front of the foot is turned inward (metatarsus varus)
Clynodactyly: bend or curvature of the fifth fingers (“pinkies") toward the adjacent fourth fingers
Coarctation of the aorta: congenital constriction (narrowing) of the aorta that impedes the flow of blood below the level of the constriction and increases blood pressure
Cockayne syndrome: inherited form of dwarfism caused by a defect in one of the genes involved in the normal repair of DNA (ERCC8 gene for CS-type I mapped to chromosome 5 and ERCC6 gene for CS-type mapped to chromosome 10 at locus 10q11); diagnosed by the presence of three signs: growth retardation (short stature), abnormal sensitivity to light (photosensitivity), and prematurely aged appearance (progeria)
Coding region: portion of a gene's DNA or RNA (exons) that codes for protein. The region is bounded nearer the 5' end by a start codon and nearer the 3' end with a stop codon
Codon: a group of three nucleotides in messenger RNA (mRNA) that represent a single amino acid in the genetic code
Coloboma: hole in one of the structures of the eye, such as the iris, retina, choroid, or optic disc
Comparative genomic hybridization (CGH): technology wherein a DNA test sample is competitively hybridized with a reference sample of DNA of known sequence to a DNA microarray, used to detect copy number changes in the test sample
Complementary base pairing: hydrogen bond formation in DNA and RNA that occurs only between cytosine and guanine (in both DNA and RNA) or between adenine and thymine (in DNA) or adenine and uracil (in RNA).
Confined placental mosaicism: mosaicism that is seen only in the placenta but not in the fetus
Congenital: present at birth, not necessarily inherited
Congenital defect: a defect or disorder that occurs during prenatal development
Conotruncal heart: general term for truncus arteriosus, transposition of the great arteries, double outlet of the right ventricle, and tetralogy of Fallot; in truncus arteriosus there is a single outflow tract instead of a separate aorta and pulmonary artery
Consanguine: of the same blood or origin; in genetics, the term implies the sharing of genetic traits or characteristics from the same ancestors (eg, cousins)
Contig: A map that shows regions on a chromosome where neighboring portions of DNA overlap; the map provides information about a large segment of a genome by looking at a series of overlapping clones
Contracture: shortening and hardening of muscles, tendons, or other tissue, often leading to deformity and rigidity of joints
Copy number variations:
Corectopia: pupil of the eye is off-center
Cornelia de Lange syndrome inherited as an autosomal dominant condition involving the NIPBL gene on chromosome 5 or as an X-linked condition involving the SMC1L1 gene on the X chromosome; characterized by delays in physical development before and after birth, characteristic abnormalities of the head and facial (craniofacial) area, malformations of the hands and arms (upper limbs); and mild to severe intellectual impairment; other symptoms include an unusually small, short head (microbrachycephaly), an abnormally long vertical groove between the upper lip and nose (philtrum), a depressed nasal bridge, upturned nostrils (anteverted nares) and a protruding upper jaw (maxillary prognathism)
Cor triatriatum: congenital heart defect in which there is a small extra chamber which results in slowing of the passage of blood from the lungs into the heart (left atrium and ventricle) due to the pulmonary veins (returning blood from the lungs) draining into this extra "third atrium"
Costo-vertebral dysplasia: genetic disorder characterized by spinal column abnormalities, dwarfism and a swollen abdomen; also called Jarcho-Levin syndrome
Cowden syndrome: disorder characterized by multiple noncancerous, tumor-like growths called hamartomas and an increased risk of developing certain cancers
Coxa valga: broadening of the head and neck of the femur
Craniofacial: relating to the cranium and the face
Craniosynostosis: a congenital defect involving the skull bones of a newborn, whereby the bones are fused together at birth and require surgical intervention to allow for normal brain development
Cri–du–chat syndrome: also called 5p minus syndrome or “Cat Cry” or syndrome due to the cat-like cry typical of newborns; chromosomal disorder in which a variable portion of the short arm of chromosome 5 is missing or deleted; most deletions are de nova or sporadic; common symptoms include a distinctive cry that resembles a cat’s mewing sound, characteristic facial features, slow growth, and microcephaly; affected children also exhibit delays in the acquisition of skills requiring the coordination of muscular and mental activities (psychomotor disability) and moderate to severe intellectual disability
Critical region: specific portion of a chromosome or a gene that, when altered in some way (deleted, duplicated, or otherwise mutated), produces a characteristic set of phenotypic abnormalities associated with a particular syndrome or disorder
Cryptophthalmos: absent palpebral fissures, with skin passing continuously from the forehead or eyebrow onto the cheek, often with the presence of a tuft of hair; typically associated with a rudimentary or small globe (hidden eye)
Cryptorchidism: absence of one or both testes from the scrotum
CT scan: computerized tomography that combines a series of X-ray views taken from many different angles with computer processing to create cross-sectional images of the bones and soft tissues
Cutaneous: relating to, or affecting the skin
Cutis marmorata: pink marble-like mottling of the skin caused by exposure to cold temperature or associated with various debilitating diseases
Cyclopia: congenital defect characterized by fusion of the orbits into a single cavity containing one eye
Cysterna magna: one of three principal openings in the subarachnoid space between the arachnoid and pia mater layers of the meninges surrounding the brain
Cystic (renal) dysplasia: abnormal tissue development in the kidney that can be confined to a small portion of one (unilateral or both (bilateral) kidneys arranged in a specific pattern within the kidney or randomly present throughout one or both kidneys
Cytogenetics: the study of chromosome number and structure, including identification of abnormalities
Cytosine (C): a pyrimidine nitrogenous base found in the structure of both DNA and RNA

Dacryostenosis: Stricture or narrowing of a lacrimal duct
Dandy–Walker malformation: brain malformation that occurs during embryonic development of the cerebellum and 4th ventricle and is characterized by underdevelopment (small size and abnormal position) of the cerebellar vermis (the middle part of the cerebellum), cystic enlargement of the 4th ventricle, and enlargement of the base of the skull (posterior fossa)
de Grouchy syndrome (distal 18q) deletion of genetic material from the long arm of chromosome 18, specifically, the distal section of 18q typically extending to the tip of the long arm of chromosome 18
Deletion: a type of chromosomal mutation in which a genetic sequence is lost from a chromosome, usually through an error in recombination
de nova mutation: new or spontaneous mutation not inherited from either parent
Deoxyribonucleic acid (DNA): the genetic material found in all cells; DNA consists of nitrogenous bases (adenine, guanine, cytosine, and thymine), sugar (deoxyribose), and phosphate
Dermatofibrosis lenticularis disseminata: dermatologic condition in which small papules or disks of increased dermal elastic tissue appear in early life; autosomal dominant inheritance
Dextrocardia: congenital defect in which the apex of the heart is situated on the right side of the body
Diaphragmatic hernia: birth defect in which there is an abnormal opening in the diaphragm, causing part of the stomach, spleen, liver, and intestines to go up into the chest cavity near the lungs
Dicentric chromosome: a chromosome with two centromeres, usually resulting from an error of recombination
Di George syndrome (22q11.2 deletion): chromosomal disorder characterized by heart abnormalities, cleft palate, distinctive facial features, recurrent infections, breathing problems, kidney abnormalities, low calcium levels in the blood, thrombocytopenia, feeding and gastrointestinal problems, or hearing loss; many have delayed growth and speech development, and learning disabilities
Digital Constriction Ring (amniotic bands): narrow segment of significantly reduced circumference of a digit
Diploid: a cell or organism with two complete sets of chromosomes; humans normally have two sets of 23 chromosomes, making them diploid
Diplopia: a pathological condition of vision in which a single object appears double
Disomy: a case in which both copies of a chromosome come from a single parent, rather than one being maternal and one being paternal
Distal: situated away from the point of attachment or origin or a central point; located away from the center of the body
DNA replication: process of making an identical copy of a section of duplex (double-stranded) DNA, using existing DNA as a template for the synthesis of new DNA strands
Dolichocephaly: elongated head; cephalic index (ratio of head width expressed as a percentage of head length) less than 76%; increased antero-posterior length of the head compared to width.
Dolichocolon: abnormally long large intestine
Dominant gene: an allele or a trait that will mask the presence of a recessive allele or trait; requires only one of the genes to be mutated for the child to be affected (50% chance)
Double helix: The coiled structure of double-stranded DNA in which strands linked by hydrogen bonds form a spiral configuration, with the two strands oriented in opposite
Double outlet right ventricle: congenital heart disease in which the aorta rises from the right ventricle (the chamber of the heart that pumps blood to the lungs), instead of from the left ventricle (the normal pumping chamber to the body).
Down syndrome (trisomy 21): a chromosomal disorder caused by an extra copy of chromosome 21; symptoms include intellectual impairment, congenital heart defects, low muscle tone (hypotonia); a tendency to keep the mouth open with protrusion of the tongue, and distinctive facial characteristics including a short, small head (microbrachycephaly), upwardly slanting eyelid folds (palpebral fissures), a depressed nasal bridge, a small nose, and a relatively flat facial profile
Dravet syndrome: intractable (uncontrollable) form of epilepsy characterized by unilateral (one-sided) clonic or tonic clonic (grand mal) seizures that may be prolonged (> 5 minutes) or progress to status epilepticus (>30 minutes); myoclonic seizures, often called myoclonic jerks, are common but not always present; most cases caused by mutations of the SCN1A gene on chromosome 2q24
Duane anomaly: disorder of eye movement in which the eye is unable to move outward (toward the ear), and in some cases, inward eye movement (toward the nose) is limited
Duodenal atresia: refers to a blockage in the hollow tube that connects the stomach to the rest of the intestine
Duplication: a type of chromosomal mutation in which a chromosome region is duplicated because of an error in recombination during prophase I of meiosis
Dwarfism: the condition of adults of short stature who are less than 50 inches in height
Dysgenesis: defective development of an organ, especially the gonads
Dysmorphism: derived from the Greek dys – meaning abnormal, disease, faulty, impaired and morphology - meaning structure or form
Dysphagia: difficulty or discomfort in swallowing, as a symptom of disease
Dysphonia: difficulty in speaking due to a physical disorder of the mouth, tongue, throat, or vocal cords
Dysplasia: abnormal cell or tissue growth or development
Dysplasia: abnormal development or growth of tissues or cells
Dysplastic: alteration in size, shape, and organization of adult cells
Dyspraxia: inability to perform coordinated movements
Dystonia: movement disorder in which involuntary muscle contractions cause twisting or repetitive movements

Ebstein’s anomaly: congenital heart defect in which parts of the tricuspid valve are abnormal
Ectoderm: outermost of the three primary germ layers of an embryo, from which the epidermis, nervous tissue, and sense organs develop
Ectopic: abnormal place or position
Ectopic cordis: congenital malformation in which the heart is abnormally located either partially or totally outside of the thorax
Ectropion: outward turning (eversion) or rotation of the eyelid; usually involves the lower eyelid
Edwards syndrome (trisomy 18): also known as trisomy 18; a chromosomal disorder in which there is an extra copy of chromosome 18; newborns commonly have small eyes, deformed ears, heart defects, and severe intellectual impairment; mortality is high
Emanuel syndrome: chromosomal disorder in which there is a supernumerary chromosome consisting of a piece of chromosome 11 attached to a piece of chromosome 22 in each cell, expressed as der(22); characterized in infancy by weak muscle tone (hypotonia), failure to thrive, distinct facial appearance, and developmental delay; severe to profound intellectual disability is common
Encephalocele: birth defect associated with skull defects characterized by partial lacking of bone fusion leaving a gap through which a portion of the brain protrudes outside the skull
Endocardial cushion defect: congenital heart condition in which the walls separating all four chambers of the heart are poorly formed or absent
Entropion: inward turning (inversion) of the eyelid margin
Epicanthal fold (epiblepharon): redundant eyelid skin pressing the eyelashes against the cornea and/or conjunctiva
Epicanthus inversus: fold of skin starting at or just below the medial aspect of the lower lid and arching upward to cover, extend in front of and lateral to the medial canthus
Epignathus: unequal conjoined twins in which the smaller, incomplete parasite is attached to the larger autosite at the lower jaw
Epileptiform: resembling epilepsy or its manifestations
Epilepsy: neurological disorder marked by sudden recurrent episodes of sensory disturbance, loss of consciousness, or convulsions, associated with abnormal electrical activity in the brain
Esophageal atresia: birth defect in which the esophagus (the tube that connects the throat with the stomach) does not develop normally and is separated into two parts, an upper and lower segment one of which end in a blind pouch; the two segments do not connect and the normal passage between the mouth and stomach does not exist
Etiology: the cause or causes of a disease or disorder
Euploid: the normal number of chromosomes for a cell or organism, 46 in the case of humans
Exencephaly: disorder wherein the brain is located outside of the skull, causing the neural tissue to gradually degenerate; usually found in embryos as an early stage of anencephaly.
Exstrophy: Eversion of a hollow organ at birth, eg, exstrophic bladder is turned inside out like a rubber glove
Exome: entire portion of the protein-coding sequences of the genome, omitting introns and noncoding DNA
Exon: a segment of a DNA or RNA molecule containing information coding for a protein or peptide sequence, usually flanked by introns
Exophthalmos: abnormal protrusion of the eyeballs
Exostose: benign outgrowth of cartilaginous tissue on a bone
Expressivity: the degree to which a genotype is expressed as a phenotype
Exudative enteropathy: diarrhea that occurs in diseases characterized by inflammation or destruction of intestinal mucosa
Eyelid, cleft: short discontinuity of the margin of the lower or upper eyelid

Face, coarse features: absence of fine and sharp appearance of brows, nose, lips, mouth and chin, usually because of rounded and heavy features or thickened skin with or without thickening of subcutaneous and bony tissues
Face, flat: absence of concavity or convexity of the face when viewed in profile
Failure to thrive: term used to describe a delay in a child's growth or development; usually applied to children up to two years of age whose growth is below the 3rd or 5th percentiles for their age
Faulty gene: a gene that does not function properly
Feingold syndrome: autosomal dominant disorder characterized by microcephaly, distal limb malformations (eg, short middle phalanges of the 2nd and 5th fingers), and esophageal/duodenal atresia; usually associated with learning disabilities are common
Finger, tapered: gradual reduction in girth of the digit from proximal to distal
Fingers, cutaneous syndactyly: soft tissue continuity in the A/P axis between two fingers that lies significantly distal to the flexion crease that overlies the metacarpophalangeal joint of the adjacent fingers
Fistula: abnormal or surgically made passage between a hollow or tubular organ and the body surface, or between two hollow or tubular organs
Fluorescence in situ hybridization (FISH): a form of molecular cytogenetics; an extremely sensitive assay for determining the presence of deletions on chromosomes, which uses a fluorescence-tagged segment of DNA that binds to the DNA region being studied
Focal cortical dysplasia: congenital abnormality where the neurons in an area of the brain failed to migrate in the proper formation in utero; malformation of cortical development, which is the most common cause of medically refractory epilepsy in the pediatric population
Fontanel: “soft spot” on the skull that remains open for the brain to develop and closes during infancy
Founder effect: reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors
Fragile site: small break or constriction of a chromosome that can be visualized after special treatment of the chromosomes (in individuals affected with Fragile X syndrome, a fragile site can often be seen on their X chromosome)
Fragile X syndrome: chromosomal disorder caused by mutations in the FMR1 gene on the X chromosome, resulting in an increased number of copies of a portion of the gene called CGG repeats; the greater the number of copies of CGG, the more likely there will be increased severity of the disorder; disorder occurs mostly in males and is characterized by moderate intellectual disability, motor and language delays, and autistic behaviors; facial features include large head, long face, prominent forehead and chin, and protruding ears
Frame-shift mutation: a DNA mutation involving the insertion or deletion of one of several nucleotides that are not in multiples of three, resulting in a shift of the codon reading frame; usually produces nonfunctional proteins. Normally, the cell reads a gene in groups of three bases. Each group of three bases corresponds to one of 20 different amino acids used to build a protein. If a mutation disrupts this reading frame, then the entire DNA sequence following the mutation will be read incorrectly.
Frontal bossing: bilateral bulging of the lateral frontal bone prominences with relative sparing of the midline

G: the abbreviation for guanine, a purine nitrogenous base found in the structure of both DNA and RNA
Gain-of-function: mutations that change the gene product such that it gains a new and abnormal function and usually have dominant phenotypes
Gardner syndrome: disorder is characterized by adenomatous polyposis, an association of numerous polyps of the colon with soft tissue tumors, mostly in the brain and thyroid; also present are osteomas, causing susceptibility to colon cancer; congenital hypertrophy of the retinal pigment epithelium, jaw and/or sebaceous cysts
Gastro–esophageal reflux (GERD): condition in which the stomach contents leak backwards from the stomach into the esophagus (the tube from the mouth to the stomach)
G-banding: series of dark and light stripes across a chromosome which are produced by treating (staining) the chromosomes with different chemicals in a laboratory
Gene expression: the combined biochemical processes (transcription and translation) that convert the linearly encoded information in the bases of DNA into the three-dimensional structures of proteins
Gene families: multiple copies of the same or similar genes in the same genome; copies can be identical and tandemly repeated, or may differ slightly and be scattered on the same or different chromosomes
Gene mapping: determining the relative locations of different genes on chromosomes
Gene pool: the complete assortment of genes present in the gametes of the members of a population that are eligible to reproduce
Gene silencing: any form of genetic regulation in which the expression of a gene is completely repressed, either by preventing transcription or after a messenger RNA (mRNA) has been transcribed
Gene therapy: any procedure that genetically alters the cells of the person to alleviate or treat the symptoms of a disease or condition
Gene: a portion of a DNA molecule containing the genetic information necessary to produce a molecule of messenger RNA (via transcription) that can then be used to produce a protein (via translation)
Genetic code: information contained in the DNA which is ‘interpreted’ by the cells to produce proteins; nucleotides which make up DNA are described by the letters A (adenine), T (thymine), C (cytosine) and G (guanine); written in a series of these letters in triplicate
Genetic counseling: a discipline concerned with analyzing the inheritance patterns of a particular genetic defect within a given family, including the determination of the risk associated with the presence of a genetic defect in future generations and options for treatment of the existing genetic defect
Genetic screening: the testing of individuals for disease-causing genes or genetic disease
Genetic testing: the use of techniques of genetics research to determine a person’s risk of developing, or status as a carrier of, a disease or other disorder
Genetics: a branch of biology involving the study of heredity
Genome: complete set of genes carried by an individual or all the DNA in the cell or in one of the organelles (eg, mitochondrion)
Genomic imprinting: a genetic phenomenon in which the phenotype associated with a particular allele depends on which parent donated the allele
Genomics: branch of genetics dealing with the study of genetic sequences, including their structure and arrangement
Genotype: the genetic characteristics of a cell or organism, expressed as a set of symbols representing the alleles present at one or more loci (sites)
Genotype-phenotype: phenotype is the constellation of observable traits; genotype is the genetic endowment of the individual. Genotype is the set of genes responsible for a particular trait, phenotype is the physical expression of that trait.
Germline mosaicism: when germ cells (sperm or egg cells) have a different genetic makeup from cells in the rest of the body
Germline mutation: heritable change in the genes of an individual’s reproductive cells, often linked to hereditary diseases
Glabella: smooth part of the forehead above and between the eyebrows
Glaucoma: condition of increased pressure within the eyeball, causing gradual loss of sight
Glomerular fibrosis: formation of fibrous tissue in the renal glomerulus as a result of repair or a reactive process
Glossoptosis: posterior displacement of the tongue into the pharynx
Goldenhar syndrome: disorder characterized by hemifacial microsomia (hypoplasia of one side of the face), epibulbar dermoid (benign tumors over the cornea and sclera), and absence or severe hypoplasia of one ear
Gonadogenesis: generation and development of the gonads (organ that produces testes or ovaries).
Gonadal dysgenesis: congenital developmental disorder of the reproductive organs
Gorlin syndrome: autosomal dominant genetic disorder characterized by a broad face, rib malformations, and high susceptibility to basal cell carcinoma, a type of skin cancer
Greig cephalopolysyndactyly syndrome: genetic disorder characterized by physical abnormalities affecting the fingers and toes and the head and facial (craniofacial) area; extra (supernumerary) fingers and/or toes (polydactyly), webbing and/or fusion of the fingers and/or toes (syndactyly) may be present; craniofacial malformations may include a large and/or unusually shaped skull, high, prominent forehead (frontal bossing), abnormally broad nasal bridge, and widely spaced eyes (ocular hypertelorism)
Guanine (G): a purine nitrogenous base found in the structure of both DNA and RNA
Gyri: convolutions on the surface of a cerebral hemisphere produced by the infolding of the cerebral cortex

Hair whorl, abnormal number: more than two clockwise hair whorls
Hair whorl, abnormal position: hair growth from a single point on the scalp in any location other than lateral to the midline and close to the vertex of the skull
Hair, frontal upsweep: upward and/or sideward growth of anterior hair (cowlick)
Hallucination: Subjectively experienced sensations or false sensory perception in the absence of an appropriate stimulus, but which are regarded by the individual as real
Hamartoma: benign tumor-like malformation resulting from faulty development in an organ and composed of an abnormal mixture of tissue elements that develop and grow at the same rate as normal elements but are not likely to compress adjacent tissue.
Hand–foot–genital syndrome: condition that affects the development of the hands and feet, the urinary tract, and the reproductive system; caused by mutations in the HOXA13 gene in chromosome 7 at 7p15.2
Haploid: refers to a cell or an organism with one set of chromosomes
Haploinsufficiency: refers to a protein produced by a single gene copy which is then insufficient for normal functioning, leading to an abnormal or diseased state; responsible for some but not all autosomal dominant disorders
Hemifacial microstomia: congenital disorder that affects the development of the lower half of the face, most commonly the ears, the mouth and the mandible
Hemihypertrophy: condition in which one side of the body or a part of one side is larger than the other
Hemivertebrae: congenital anomaly of the spine that is characterized by the formation of half of a vertebral body, a corresponding pedicle, and a corresponding hemilamina
Heredity: the overall mechanism by which characteristics or traits are passed from one generation of organisms to the next; genetics is the scientific study of heredity
Heterochromia irides: difference in color between the irises of the two eyes
Heterogeneous: not uniform in structure or composition; phenomenon in which a single phenotype or genetic disorder may be caused by any one of a multiple number of alleles or non-allele (locus) mutations
Heterotaxy: abnormality where the internal thoraco-abdominal organs demonstrate abnormal arrangement across the left-right axis of the body
Heterotopia: clusters of neurons placed in unusual places along the lateral ventricles
Heterozygote: an individual with two different alleles at a gene locus (site)
Heterozygous: composed of two alleles that are different
Hexadactyly: presence of an extra digit (ie, a sixth finger or toe)
Hippocampus: elongated ridges on the floor of each lateral ventricle of the brain, thought to be the center of emotion, memory, and the autonomic nervous system
Hirschsprung’s disease: characterized by the absence of particular nerve cells (ganglions) in a segment of the bowel, causing muscles to lose their ability to move the stool through the intestine (peristalsis) and resulting in constipation and obstruction of the colon
Hirsutism: excessive growth of facial or body hair in women
Histone: specialized protein in eukaryotic cells that binds to DNA molecules and causes them to become more compact; also thought to be involved in regulation of gene expression
Holoprosencephaly (HPE): failure of the forebrain (prosencephalon) to develop normally; instead of the normal complete separation of the left and right halves of the forebrain, there is an abnormal continuity between the two sides; alobar HPE is the most severe form
Homologous: refers to chromosomes that are identical in terms of types of genes present and the location of the centromere
Homozygote: an individual with two identical alleles at a gene locus (site)
Homozygous: characterized by a genotype composed of two alleles that are the same and at the same locus
Horseshoe kidney: congenital partial fusion of the kidneys resulting in a horseshoe shape
Human Genome Project: multiyear genetic research endeavor that sequenced the entire human genome; officially completed in 2003
Hunter–McAlpine syndrome: dominantly inherited syndrome characterized mainly by premature fusion of skull bones, mental retardation, small mouth, short stature and finger and toe anomalies; thought to be caused by duplication of the region 5q35-qter on chromosome 5
Hybridization: a process of base pairing involving two single-stranded nucleic acid molecules with complementary sequences; the extent to which two unrelated nucleic acid molecules will hybridize is often used as a way to determine the amount of similarity between the sequences of the two molecules
Hydrocephalus: condition in which abnormally widened (dilated) cerebral spaces in the brain (ventricles) inhibit the normal flow of cerebrospinal fluid (CSF), resulting in accumulation of cerebrospinal fluid (CSF) in the skull which puts pressure on the brain tissue; treatment involves insertion of a shunt to redirect the CSF for elimination, usually through the stomach
Hydrocolpos: accumulation of mucus or other non- sanguineous fluid in the vagina
Hydronephrosis: distention of the kidney with urine; caused by obstruction of urine outflow
Hygroma: sac, cyst, or bursa distended with a fluid
Hyperacusia: over-sensitivity to loud noises
Hypercalcemia: abnormally high level of calcium in the blood, usually more than 10.5 milligrams per deciliter of blood
Hyperopia: condition of the eye where incoming rays of light reach the retina before they converge into a focused image
Hyperphagia: over–eating, sometimes with insatiable appetite
Hyperpigmentation: darkening of an area of skin or nails caused by increased melanin
Hypertelorism: abnormally increased distance between two organs or parts; usually refers to the eyes
Hyperthyroidism: over active thyroid gland
Hypertrichosis: abnormal amount of hair growth over the body
Hypodontia: congenital condition marked by a less than normal number of teeth
Hypogonadotropic-hypogonadism: pubertal failure similar to Kallman syndrome, but with a normal sense of smell; due to deficiency of the reproductive hormone gonadotropin-releasing hormone
Hypomelanosis of Ito: not a specific entity but rather represents features of many different forms of mosaicism; characterized by unilateral or bilateral hypopigmented macules in whorls, streaks, and patches in a “marble-cake” pattern, variably associated with epidermal nevi, alopecia, and ocular, skeletal, and neural abnormalities
Hypoparathyroidism: Underfunctioning of the parathyroid glands with deficient production of the hormone parathormone which leads to low blood calcium
Hypoparathyroidism, deafness, renal syndrome: autosomal dominant disorder characterized by hypoparathyroidism, sensorineural deafness, and renal disease caused by mutation of the GATA3 gene located on chromosome 10 at 10p15
Hypopigmentation: diminished pigmentation in a bodily part or tissue
Hypoplasia: incomplete development or underdevelopment of an organ or tissue
Hypoplastic: abnormal deficiency of cells or structural elements
Hypoplastic left heart syndrome: form of congenital heart disease in which the whole left half of the heart (including the aorta, aortic valve, left ventricle and mitral valve) is underdeveloped (hypoplastic).
Hypoplastic supraorbital ridges: Flatness of the supraorbital portion of the frontal bones (brow area)
Hypospadias: developmental anomaly of the urethra in which a part of the urethral canal is open on the undersurface of the penis or on the perineum
Hypotelorism: abnormally decreased distance between two organs or parts. Usually refers to the eyes
Hypothyroidism: underactive or inactive thyroid gland
Hypotonia: floppiness, low or underdeveloped muscle tone

Ichthyosis: general term for a family of genetic skin diseases characterized by dry, thickened, and scaling skin; various forms are distinguished from one another by extent of scaling and how widely and where the scaling is over the body, by presence or absence and intensity of reddening of the skin (erythroderma), by the mode of inheritance, and by the character of associated abnormalities
Immunoglobulin A (IgA): antibody that plays a critical role in mucosal immunity
Imperforate anus: inborn abnormality characterized by the absence (anal atresia) or abnormal localization of the anus; rectum or colon may be connected to the vagina or the bladder by a fistula
Imprinting: ‘memory’ held by a chromosome as to whether it was inherited from the mother or the father and chemically ‘stamped’ into the DNA; results in chromosomes or genes located on the chromosomes that behave differently, depending on the parent of origin; faulty genes on chromosome 15 cause both Angelman (from the mother) and Prader-Willi syndromes (from the father)
Incidence: the frequency in which a disorder appears in a given population
Infantile spasms: seizure disorder of infancy and early childhood with the onset predominantly in the first year of life characterized by myoclonic seizures, hypsarrhythmia (abnormal, chaotic electroencephalogram), and mental retardation
Informed consent: the right of patients to know the risks of medical treatment and to determine what is done to their bodies, including the right to accept or refuse treatment based on this information; usually refers to participation in a clinical trial
Infra-orbital folds: Elevated ridge(s) of skin starting well below the medial aspect of the lower lid and curves gradually upward toward and/or across the nasal bridge
Inherited: genetic transfer from parents to children
Initiation codon: also called the “start codon,” a codon, composed of the nucleotides AUG, that signals the beginning of a protein-coding sequence in a messenger RNA (mRNA) molecule; in the genetic code, AUG always represents the amino acid methionine
Insertion: type of mutation whereby a DNA sequence is abnormally inserted into a gene, disrupting the normal function and structure of the gene and leading to a chromosomal abnormality
Interrupted aortic arch: congenital heart malformation in which a small section of the aorta is missing, causing a loss of luminal continuity between the ascending and descending portions of the aorta.
Interstitial deletion: deletion that does not involve the terminal parts (ends) of a chromosome
Intestinal atresia: broad term used to describe a complete blockage or obstruction anywhere in the intestine
Intestinal malrotation: congenital anomaly of rotation of the midgut in which the gut undergoes a complex rotation outside the abdomen during development
Intrachromosomal duplication: translocation that occurs within a chromosome
Intragenic: being or occurring within a gene
Intron: a segment of a DNA or RNA molecule that does not code for proteins and interrupts a gene-coding sequence or nontranslated sequence; segment of a gene situated between exons that is removed before translation of messenger RNA and does not function in coding for protein synthesis.
Inversion: a chromosomal abnormality resulting in a region of the chromosome where the normal order of genes is reversed or the segment of the chromosome has two breaks and “flips over”; both instances can cause the genetic code to be translated (read) wrong
Isochromosome: chromosome that has lost one of its arms and replaced it with an exact copy of the other arm
Isodicentric: extra piece of chromosome that has been duplicated end-to-end like a mirror image
Isodisomy: two identical copies of one of the parental alleles

Jacobsen syndrome: 11q deletion syndrome in which the deletion occurs at the end (terminus) of the long (q) arm of chromosome 11; characterized by delayed development in motor and speech, cognitive impairment, ADHD-like behavior, distinctive facial features, and the bleeding disorder Paris-Trousseau syndrome

Kallman syndrome: genetic disorder defined by delay/absence of signs of puberty along with an absent/impaired sense of smell; caused by isolated deficiency of a key reproductive hormone called gonadotropin-releasing hormone
Karyotype: the complete set of chromosomes possessed by an individual, usually isolated during metaphase and arranged by size and type as a method of detecting chromosomal abnormalities
Keratoconus: non-inflammatory eye (ocular) condition characterized by progressive changes in the shape of the cornea
Kyphoscoliosis: combination of outward curvature (kyphosis) and lateral curvature (scoliosis) of the spine

Lacrimal punctum, absence: No identifiable superior and/or inferior lacrimal punctum (tear ducts)
Langer–Giedion syndrome: genetic condition that causes bone abnormalities (exostoses, short stature and cone-shaped ends of the long bones) and distinctive facial features (sparse scalp hair, rounded nose, long flat philtrum, and thin upper lip); caused by at least two genes on chromosome 8
Leukopenia: abnormally low number of white blood cells (leukocytes) in the blood circulation
Leukocoria: white pupillary reflex in which the pupil looks white instead of red when a bright light is shone it; occurs with congenital cataracts
Library: a cloned collection of DNA created from a specific organism
Lipoma: common benign tumor usually found in the fatty layer of the skin
Lissencephaly: smooth brain; brain malformation characterized by absence (agyria) or incomplete development (pachygyria) of the ridges or convolutions (gyri) of the outer region of the brain (cerebral cortex); occurs as an isolated abnormality or in association with certain underlying syndromes (eg, Miller-Dieker syndrome)
Locus (pl. loci): the specific location of a particular gene on a chromosome
Lordosis: increased inward curving of the lumbar spine
Loss of function: gene product having less or no function. When the allele has a complete loss of function (null allele) it is often called an amorphic mutation. Phenotypes associated with such mutations are most often recessive.
Low set ears: upper insertion of the ear to the scalp below an imaginary horizontal line passing through the inner canthi and extending that line posteriorly to the ear
Lymphedema: condition in which excess fluid collects in tissue and causes swelling
Lionization: see X-inactivation

Macrocephaly: occipitofrontal (head) circumference greater than 97th centile compared to appropriate, age matched, sex-matched normal standards
Macular retinal dystrophy: group of disorders involving predominantly the posterior portion of the ocular fundus; due to degeneration in the sensory layer of the retina, retinal pigment epithelium, Bruch membrane, choroid, or a combination of these tissues.
Malformation: an inherited organ defect that occurs during fetal development
Mandible, cleft: midline deficiency of the mandible and some or all overlying tissues
Marker chromosome: structurally abnormal chromosome in which no part can be identified
Medial: term meaning toward the median plane (point in the centre of the organism (where the left-right axis intersects the midsagittal plane)
Medulla: innermost part
Megalocornea: largeness of the corneas
Melatonin: hormone that is produced by the pineal gland and is intimately involved in regulating the sleeping and waking cycles
Meningocele: protrusion of the membranes that cover the spine and part of the spinal cord through a bone defect in the vertebral column
Mesencephalon: part of the brain developed from the middle of the three primary vesicles of the embryonic neural tube, comprising the tectum and the cerebral peduncles.
Mesomelic dysplasia: deformity of the middle parts of a limb; disorder of bone growth in which the long bones in the arms and legs are extremely shortened
Messenger RNA (mRNA): a type of RNA molecule containing the genetic information necessary to produce a protein through the process of translation; produced from the DNA sequence of a gene in the process of transcription
Metacentric chromosome: a chromosome with the centromere located at or near the middle of the chromosome
Metatarsus adductus: metatarsals of the foot are deviated medially (tibially)
Metencephalon: anterior segment of the developing vertebrate hindbrain
Metopic suture: persistent frontal suture, sometimes discernible a short distance above sutura frontonasalis
Microarray: flat surface on which 10,000 to 100,000 tiny spots of short DNA molecules (oligonucleotides) are fixed and are used to detect the presence of DNA or RNA molecules that are homologous to the oligonucleotides
Microcephaly: occipito-frontal (head) circumference (OFC) less than 3rd centile compared to appropriate, age matched, normal standards; decreased size of the cranium and marked posterior sloping of the forehead
Microdeletion: loss of a tiny piece from a chromosome that is too small to be seen readily through a microscope
Microdontia: condition in which a single tooth, pairs of teeth, or the whole dentition, are disproportionately small
Microduplication: gain (repeat) of a tiny piece from a chromosome that is too small to be seen readily through a microscope
Micrognathia: small chin; reduced length and width of the mandible when viewed from the front but not from the side; shortening and narrowing of the mandible and chin
Micromelia: Condition of having disproportionately short or small limbs
Microphthalmia: congenital malformation of the globe, a birth defect of the eye
Microstomia: abnormal smallness of the mouth
Microtia: small ear; some ear components may be missing or disfigured; median longitudinal length of the ear is more than 2 standard deviations below the mean median longitudinal length;
Midface retrusion (midface hypoplasia): posterior positioning and/or vertical shortening of the infraorbital and perialar regions, or increased concavity of the face and/or reduced nasolabial angle; underdevelopment of the maxilla (upper jaw) in length (decreased midface height) or depth
Miller–Dieker syndrome: condition characterized by a pattern of abnormal brain development (lissencephaly), causing severe intellectual disability, developmental delay, seizures, abnormal muscle stiffness (spasticity), weak muscle tone (hypotonia), feeding difficulties, and distinctive facial features; caused by caused by a deletion of genetic material near the end of the short (p) arm of chromosome 17
Missense mutation: single base pair substitution (point mutation) that results in the translation of a different amino acid at that position, usually rendering the resultant protein nonfunctional because the base change or substitution results in a codon that causes insertion of a different amino acid
Molecular genetics: the branch of genetics concerned with the central role that molecules, particularly the nucleic acids DNA and RNA, play in heredity
Monosomy: a genetic condition in which one chromosome from a homologous chromosome pair is missing, producing a 2n-1 genotype; usually causes significant problems in phenotype, when a particular chromosome appears only once instead of twice in cells
Morphology: science of structure and form of organisms without regard to function
Mosaicism: a condition in which an individual has two or more cell populations derived from the same fertilized ovum, or zygote; some cells have an abnormal or unusual genetic or chromosomal makeup while the rest of the cells in the body have the usual genetic or chromosomal constitution
Mouth, downturned corners (carp mouth): oral commissures positioned inferior to the midline labial fissure
Mowat–Wilson Syndrome: genetic disorder characterized by intellectual disability, distinctive facial features and seizures; other congenital anomalies occur in some individuals, including Hirschsprung’s disease (40-50% of individuals), heart (cardiac) defects, kidney (renal) abnormalities, male genital abnormalities and short stature; caused by an abnormality in the ZEB2 gene located on chromosome 2 at 2q22.3 and is usually the result of a new genetic change (mutation) in the affected person
MRI: Magnetic resonance imaging
Müllerian ducts: pair of ducts parallel to the Wolffian ducts and giving rise to the fallopian tubes, uterus, cervix, and upper portion of the vagina
Mutant: a trait or organism different from the normal, or wild-type, trait or organism seen commonly in nature; mutants can arise either through expression of particular alleles in the organism or through spontaneous or intentional mutations in the genome.
Mutation: a change in the genetic sequence of an organism, usually leading to an altered phenotype
Myelin: soft white material of lipid and protein that is secreted by oligodendrocytes and Schwann cells and forms a thick sheath about axons
Myelin sheath: The cover that surrounds many nerve cells and helps to increase the speed by which information travels along the nerve
Myelination: development, or formation of a myelin sheath around a nerve fiber
Myoclonic seizure: seizure characterized by jerking (myoclonic) movements of a muscle or muscle group, without loss of consciousness
Myoclonus dystonia: movement disorder that typically affects the upper half of the body such that quick, involuntary muscle jerking or twitching (myoclonus) occurs, usually restricted to the arms, neck, and trunk
Myopia: nearsightedness

Nail, bifid: digit with two nails, with at least some soft tissue between them
Nares, anteverted: anteriorly-facing nostrils viewed with the head in the Frankfurt horizontal and the eyes of the observer level with the eyes of the subject; tip of the nose is upturned and is positioned superiorly to the nasal base, allowing the nares to be easily visualized from the front
Naris, single: one external opening of the nose; may occur in the midline or on one side and be symmetrical or asymmetrical
Naris, supernumerary: more than two nares
Nasal bridge, depressed: posterior positioning of the nasal root in relation to the overall facial profile for age
Nasal bridge, wide: increased breadth of the nasal bridge
Nasal cartilage, absent: lack of a palpable nasal cartilage; there may also be deficiency of the nasal bone, often related to holoprosencephaly
Nasal tip, overhanging: positioning of the nasal tip inferior to the nasal base
Neck webbing: paravertically oriented fold of skin on the postero-lateral aspect of the neck, usually extending from the mastoid region of the skull to the acromion, most noticeable in frontal or posterior view
Nephroblastomatosis: clusters of microscopic blastema cells, tubules, and stromal cells at the periphery of the renal lobes in an infant; believed to be a precursor of Wilms’ tumor
Neural tube: the embryonic precursor to the spinal cord and brain, which normally closes at small openings, or neuropores, by the twenty-eighth day of gestation in humans
Neuroblastoma: childhood form of cancer that arises in the adrenal gland or in tissue in the nervous system that is related to the adrenal gland
Neurofibromatosis I (NF1) genetic disorder characterized by the development of multiple noncancerous (benign) tumors of nerves and skin (neurofibromas) and areas of abnormally decreased or increased coloration (hypo- or hyperpigmentation) of the skin; areas of abnormal pigmentation typically include pale tan or light brown discolorations (cafe-au-lait spots) on the skin of the trunk and other regions as well as freckling, particularly under the arms and in the groin; caused by mutations of gene on the long arm (q) of chromosome 17 at 17q11.2
Neurofibromatosis type II: characterized by benign (noncancerous) tumors of the nerves that transmit sound impulses from the inner ears to the brain; associated symptoms may include disturbances of balance and walking (gait), dizziness, headache, facial weakness or numbness, ringing in the ears (tinnitus), and/or progressive hearing loss; caused by mutations of a gene on the long arm (q) of chromosome 22 at 22q12.2
Noncoding DNA: a strand of DNA that is unable to make protein because it lacks the necessary information
Nondisjunction: the improper division of chromosomes during anaphase of mitosis or meiosis, resulting in cells with abnormal numbers of chromosomes and sometimes seriously altered phenotypes
Nonhistone proteins: a heterogeneous group of acidic or neutral proteins found in chromatin that may be involved with chromosome structure, chromatin packaging, or the control of gene expression.
Nonsense codon: another term for a termination or stop codon (UAA, UAG, or UGA).
Nonsense mutation: a DNA mutation that changes an existing amino acid codon in a message to one of the three termination, or stop, codons; this results in an abnormally short or truncated protein that is usually nonfunctional.
Nose, absent: complete absence of all nasal structures
Nose, bifid: visually assessable vertical indentation, cleft, or depression of the nasal bridge, ridge and tip
Nuchal skin, Redundant: excess skin around the neck, often lying in horizontal folds
Nucleic acid: the genetic material of cells, found in two forms: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA); composed of repeating subunits called nucleotides
Nucleoside: a building block of nucleic acids, composed of a sugar (deoxyribose or ribose) and one of the nitrogenous bases: adenine (A), cytosine (C), guanine (G), thymine (T), or uracil (U).
Nucleotide: a building block of nucleic acids, composed of a sugar (deoxyribose or ribose), one of the nitrogenous bases (adenine, cytosine, guanine, thymine, or uracil) and one or more phosphate groups
Nullisomy: having two less than the diploid number of chromosomes due to loss of one chromosome pair
Nystagmus: Rapid, rhythmic, repetitious, and involuntary eye movements

Occiput, flat: reduced convexity of the occiput (posterior part of skull), giving the appearance of flattening of the back of the skull.
Occiput, prominent: increased convexity of the occiput (posterior part of the skull), giving the appearance of prominence at the back of the skull
Okihiro syndrome: association of Duane syndrome (eye retraction) with forearm malformation and deafness; autosomal dominant disorder due to mutation of gene located in chromosome region 20q13.13-q13.2
Oligodactyly: presence of fewer than five fingers or toes on a hand or foot
Oligohydramnios: deficiency of amniotic fluid which compresses the fetus
Oligonucleotide: a short molecule of DNA, generally fewer than 20 bases long and usually synthesized artificially; an important tool for numerous molecular biology procedures, including site-directed mutagenesis
Omphalocele: birth defect in which an infant's intestine or other abdominal organs are outside of the body because of a hole in the navel
Open reading frame (ORF): a putative protein coding DNA sequence, marked by a start codon at one end and a stop codon at the other end.
Orbeli syndrome: distal 13q deletion characterized by retinoblastoma, mental and growth retardation, brain malformations, heart defects, distal limb deformities, and digestive, urogenital abnormalities, especially when band q32 is involved
Osseous Syndactyly of the foot: Lateral (A/P) fusion of the digits (phalanges and/or metatarsals) by hard tissue (cartilage and/or bone)
Ossification: process of bone formation
Osteopoikilosis: inherited condition characterized by bones that are mottled because of widespread small foci of compact bone in the spongy bone

p arm: short region of a chromosome attached to the centromere at the top end and the telomere at the other; contains bands of genes numbered from the centromere out
Pachygiria: unusually thick convolutions of the cerebral cortex
Palate, Hard, Short: distance between the labial point of the incisive papilla to the midline junction of the hard and soft palate more than 2 SD below the mean; decreased length of the hard palate
Palate, high: height of the palate more than 2 SD above the mean; palatal height at the level of the first permanent molar more than twice the height of the teeth
Palate, submucous cleft: soft palatal defect with intact overlying mucosa comprising two of the following three findings: (1) notching of the posterior border of the hard palate, (2) bifid uvula, or (3) muscular diastasis leading to a midline translucent zone or furrow in the soft palate
Pallister-Killian syndrome (Tetrasomy 12p): chromosomal disorder caused by the presence of four copies of the short arm of chromosome 12; symptoms may include a coarse face with a high forehead, sparse hair on the scalp, an abnormally wide space between the eyes, a fold of the skin over the inner corner of the eyes, a broad nasal bridge with a highly arched palate, and streaks of skin lacking color; intellectual impairment and loss of muscle tone are common
Palmar crease, absent; absence of a major crease of the palm (distal transverse crease, proximal transverse crease, or thenar crease)
Palpebral fissure, down-slanted: inclination is more than 2 SD below the mean for age
Palpebral fissure, up-slanted: inclination is more than 2 SD above the mean for age; may be associated with microcephaly
Palpebral fissure: scaphoid space or outline formed by eyelid margins when the eye is open
Pancytopenia: abnormal reduction in the number of red blood cells, white blood cells, and blood platelets in the blood
Paracentric inversion: an inversion of a chromosome’s sequence that does not involve the centromere, taking place on a single arm of the chromosome
Paranoia: unfounded or exaggerated distrust of others, sometimes reaching delusional proportions
Parenchyma: essential or functional elements of an organ
Parietal foramina: inconstant foramen in the parietal bone occasionally found bilaterally near the sagittal margin posteriorly; when present it transmits an emissary vein to the superior sagittal sinus
Paroxysm: sudden attack or increase of symptoms of a disease
Partial trisomy: occurs when only a part of a chromosome attaches to another
Patau syndrome (trisomy 13): genetic disorder in which there is an extra copy of chromosome 13; generally causes malformed, low set ears, severe intellectual impairment, kidney and heart defects, and microcephaly. Mortality is high
Patent ductus arteriosus: congenital heart in which the ductus arteriosus does not close
Patent foramen ovale: hole between the left and right atria (upper chambers) of the heart that did not close before birth
Pathologic: altered or caused by disease; indicative of disease
Pectus carinatum: protrusion of the sternum
Pectus excavatum: inward malformation of the sternum
Pedigree: diagrammatic representation of a family health history or family health tree
Pericentric inversion: an inversion of a chromosome’s sequence involving the centromere
Periventricular heterotopia: brain malformation, due to abnormal neuronal migration, in which a subset of neurons fails to migrate into the developing cerebral cortex and remains as nodules that line the ventricular surface; X-linked dominant disorder mapped to FLN1gene; females display borderline mental retardation, epilepsy of variable severity, and extra-central nervous system signs, especially cardiovascular defects or coagulopathy.
Pes cavus: presence of an unusually high plantar arch
Pes Planus (flat feet): arch of the foot is in contact with the ground or floor when the individual is standing
Peters’ anomaly: congenital eye disorder in which development of the anterior segment is abnormal, leading to incomplete separation of the cornea from the iris or the lens
Peutz–Jeghers syndrome: autosomal dominant genetic condition characterized by multiple benign polyps called hamartomas on the mucous lining in the gastrointestinal system and dark skin discolorations, especially around the eyes, nostrils, mucous membranes of the mouth, perianal area, and inside the mouth
Phalange: bones of the fingers and toes
Phelan–McDermid syndrome: chromosomal disorder in which a portion of the long arm (q) of chromosome 22 is missing; characterized by low muscle tone, normal to accelerated growth, absent to severely delayed speech, moderate to profound intellectual impairment, and minor dysmorphic features
Pena-Shokeir syndrome: autosomal recessive non-aneuploidic condition with some clinical features similar to that of trisomy 18; most often it is lethal
Phenotype: the physical appearance or biochemical and physiological characteristics of an individual, which is determined by both heredity and environment
Philtral ridges, malaligned: absence of the usual parallel position of philtral ridges
Philtrum, deep: accentuated, prominent philtral ridges giving rise to an exaggerated groove in the midline between the nasal base and upper vermillion border
Philtrum, long: distance between nasal base and midline upper lip vermilion border more than 2 SD above the mean; increased distance between nasal base and midline upper lip vermilion border; length of philtrum usually shows accompanied by a short nose
Philtrum: vertical groove in the midline portion of the upper lip bordered by two lateral ridges or pillars
Phonophobia: persistent, abnormal, and unwarranted fear of sound
Piebaldism: Patchy absence of the pigment of scalp hair, giving a streaked appearance
Pierre Robin complex: congenital anomaly characterized by an unusually small lower jaw (micrognathia), downward displacement of the tongue (glossoptosis), and occasionally, cleft palate
Pinna: part of the ear that projects like a little wing from the head
Pitt–Hopkins syndrome characterized by severe intellectual disability and developmental delay, breathing problems, recurrent seizures (epilepsy), and distinctive facial features and caused by mutations in the TCF4 gene on chromosome 18 at 18q21.1
Plagiocephaly: asymmetric head shape, which is usually a combination of unilateral occipital flattening with ipsilateral frontal prominence, leading to a rhomboid cranial shape
-ploid, -ploidy: a suffix that refers to a chromosome set; humans have two sets of chromosomes and are referred to as being diploid
Point mutation: a DNA mutation involving a single nucleotide
Politelia: presence of supernumerary (extra) nipples
Polycystic Kidney Disease: autosomal recessive genetic disorder caused by mutations of the PKHD1 gene located on chromosome 16 at 16p13.3 and characterized by the formation of fluid-filled sacs (cysts) in the kidneys; additional organ systems may be affected, especially the liver; high blood pressure (hypertension), excessive thirst, frequent urination and feeding difficulties may also occur
Polydactyly, mesoaxial: presence of a supernumerary finger or toe (not a thumb or hallux) involving the third or fourth metacarpal/tarsal with associated osseous syndactyly
Polydactyly, Mirror Image: hand or foot with more than five digits that has a recognizable A/P axis of symmetry.
Polydactyly: a genetic disorder where the embryo develops extra fingers or toes
Polymicrogyria: developmental anomaly of the brain marked by development of numerous small convolutions (microgyri), causing cognitive impairment
Polymorphism: the presence of many different alleles for a particular locus in individuals of the same species
Polyploid: a cell or organism that possesses multiple sets of chromosomes
Polyposis: condition characterized by the presence of numerous polyps
Postaxial polydactyly of the foot: presence of a supernumerary digit that is not a hallux (big toe)
Postaxial: pertaining to or situated behind the axis of the body, especially the posterior side
Posterior embryotoxon: ring-like opacity at the margin of the cornea in the eye
Posterior: toward the back or denoting the back surface of the body
Potocki-Lupski syndrome: caused by duplication on chromosome 17p11.2; wide range of developmental and motor issues; strong association with Charcot-Marie-Tooth disease
Potocki–Shaffer syndrome: disorder that affecting development of the bones, brain, and other organs, characterized by the growth of exostoses, enlarged parietal foramina, intellectual disability, delayed development of speech, motor and social skills, and distinctive facial features; caused by deletions on the short (p) arm of chromosome 11 at 11p11.2
Potter sequence: kidneys fail to develop properly, causing lack of amniotic fluid (oligohydramnios) that, in turn causes pressure of the uterine wall which leads to an unusual facial appearance and stops development of the lungs; can also lead to abnormal limbs
Prader–Willi syndrome: imprinting chromosomal disorder caused by mutation of chromosome 15 in the region of 15q11.2-q13 on the paternal side (mutation of this region on the maternal side causes Angelman syndrome); characterized by low muscle tone and poor feeding in infancy and an insatiable appetite as the child grows; varying degrees of cognitive impairment are common
Preauricular pits: Small indentation in front of the ear opening
Preaxial polydactyly of the hand: Duplication of all or part of the first ray
Preaxial: Situated in front of or superior to the median axis of the body or a body part
Prenatal hypoplasia: low birth weight when born at term
Prenatal testing: testing that is done during pregnancy to examine the chromosomes or genes of a fetus to detect the presence or absence of a genetic disorder.
Prevalence: the percentage of people in a given population that are affected by a disorder at that particular time
Proband: affected person whose disorder, or concern about the disorder, brings a family or pedigree to be genetically evaluated
Proboscis: fleshy, tube-like structure usually located in the midline of the face or just to one side of the midline; has a single cavity and may be found in association with a malformed nose and absence of one or both nares
Propositus: the individual in a human pedigree who is the focus of the pedigree, usually by being the first person who came to the attention of the geneticist
Prosencephalon: another term for forebrain
Protein: a biological molecule composed of amino acids linked together by peptide bonds; used as structural components of the cell or as enzymes; the term “protein” can refer to a single chain of amino acids or to multiple chains of amino acids functioning in a concerted way
Proximal: situated nearer to the center of the body or the point of attachment
Psycho-motor development: Of or relating to movement or muscular activity associated with mental processes
Pterygia: wing–like triangular membrane between the torso and upper arm in the armpit area
Pterygium colli: redundant neck skin
Ptosis: upper eyelid lid margin is positioned 3mm or more lower than usual and covers the superior portion of the iris; upper lid margin obscures at least part of the pupil; reduction in eyelid opening that is not fixed but can be increased actively or passively
Pubertas praecox: Sexual development before the age of eight in girls, and age 10 in boys
Pulmonary hypoplasia: incomplete development of the lungs, resulting in an abnormally low number or size of bronchopulmonary segments or alveoli.
Pulmonary stenosis: condition in which the pulmonary valve is too tight, so that the flow of blood from the right ventricle of the heart into the pulmonary artery is impeded
Purine: either of the nitrogenous bases adenine or guanine; used in the structure of nucleic acids.
Pyelocalyceal: Relating to the renal pelvis and calices
Pyeloectasis: dilation of the renal pelvis
Pyloric stenosis: abnormal narrowing of the valve at the outlet from the stomach, preventing normal passage of food into the small intestine
Pyrimidine: any of the nitrogenous bases cytosine, thymine, or uracil; used in the structure of nucleic acids.

q arm: long region of a chromosome attached to the centromere at the bottom and the telomere at the other end contains bands of genes numbered from the centromere out
qter: meaning terminal end of a chromosome arm; to the end

Radio-ulnar synostosis: abnormal connection between the radius and ulna, the two bones of the forearm
Ray, absent: absence of all phalanges of a digit and the associated metacarpal/metatarsal
Reading frame: refers to the manner in which a messenger RNA (mRNA) sequence is interpreted as a series of amino acid codons by the ribosome; because of the triplet nature of the genetic code, a typical messenger RNA (mRNA) molecule has three possible reading frames, although usually only one of these will actually code for a functional protein
Recessive: a term referring to an allele or trait that will only be expressed if another, dominant, trait or allele is not also present
Reciprocal deletion: recombination products of crossing-over are a duplication at the site of the exchange and a reciprocal deletion
Reciprocal translocation: a two-way exchange of genetic material between two nonhomologous chromosomes, resulting in a wide variety of genetic problems depending on which chromosomes are involved in the translocation
Recombination: formation of new set of alleles on a single chromosome that is not the same as either parent owing to a crossover during meiosis
Renal: of the kidneys
Renal agenesis: absence of one or both kidneys
Rethore syndrome (trisomy 9p): characterized by cognitive impairment, distinctive malformations of the skull and facial (craniofacial) region, skeletal defects, and structural malformations of the heart; caused by duplication of the short arm (p) of chromosome 9
Retinitis pigmentosa: group of inherited vision disorders that cause progressive degeneration of the retina, the light sensitive membrane that coats the inside of the eyes; may be associated with deafness, obesity, kidney disease, central nervous system and metabolic disorders, and occasionally chromosomal abnormalities
Retinoblastoma: malignant tumor that develops in the nerve-rich layers that line the back of the eyes (retina)
Retrognathia: posteriorly positioned lower jaw, which is set back from the plane of the face when viewed from the side but not from the front; may be accompanied by micrognathia
Reverse transcription polymerase chain reaction (RT-PCR): a technique, requiring isolated RNA, for quickly determining if a gene or a small set of genes are transcribed in a population of cell
Rhizomelic: disproportion in the length of the most proximal segment of the limbs (upper arms and thighs)
Ribonucleic acid (RNA): a form of nucleic acid in the cell used primarily for genetic expression through transcription and translation; in structure, it is virtually identical to DNA, except that ribose is used as the sugar in each nucleotide and the nitrogenous base thymine is replaced by uracil; present in three major forms in the cell: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)
Rieger syndrome: genetic disorder characterized by absent or underdeveloped (hypodontia or partial adontia) teeth, mild craniofacial abnormalities, and various abnormalities of the eye, especially glaucoma
Ring chromosome: occurs as a result of the fusion of the two ends of the same chromosome; there is a consequent loss of genetic material.
Ritscher–Schinzel syndrome (craniocerebellocardiac dysplasia): characterized by heart defects, cerebellar hypoplasia, and cranial dysmorphism; autosomal recessive disease caused by mutation on the long arm (q) of chromosome 8 at 8q24.
Robertsonian translocation: type of translocation exclusive to the acrocentric chromosomes (13, 14, 15, 21 and 22) in which two of these chromosomes join at or near their centromeres, constituting a fusion between two whole chromosomes
Rocker bottom feet: presence of both a ‘prominent heel’ and a ‘convex contour of the sole’
Rubinstein–Taybi syndrome: multisystem disorder characterized by growth retardation and delayed bone age, developmental and psycho-motor delays, cognitive impairment, distinctive abnormalities of the head and face (craniofacial dysmorphism), and/or breathing and swallowing difficulties; many may have malformations of the heart, kidneys, urogenital system, and/or skeletal system; gene responsible is located on the short arm (p) of chromosome 16 at 16p13.3
Russell -Silver syndrome: disorder characterized by prenatal and postnatal growth retardation, large forehead that protrudes out from the plane of the face, triangular-shaped face, pinky that is fixed or "locked" in a bent position (clinodactyly), lack of appetite/low BMI, and undergrowth of one side or limb(s) of the body (hemihypotrophy), resulting in unequal (asymmetric) growth; heterogeneous, but believed to be caused by abnormalities certain genes on chromosomes 7 or 11

Saethre–Chotzen syndrome: "acrocephalosyndactyly" disorder characterized by anomalies of the craniofacial area and hands and feet including craniofacial asymmetry, widely spaced eyes (ocular hypertelorism) with unusually shallow eye cavities (orbits), drooping of the upper eyelids (ptosis), and strabismus; occasional "beaked" nose, deviated nasal septum, small, low-set ears; or underdeveloped upper jaw (hypoplastic maxilla), partial fusion of soft tissues (cutaneous syndactyly) of certain fingers and toes (digits); unusually short digits (brachydactyly); and broad great toes; caused by mutations in the TWIST1 gene located on the short arm (p) of chromosome 7 at 7p21; autosomal dominant inheritance;
San Luis Valley Syndrome: recombinant 8 syndrome; characterized by heart and urinary tract abnormalities, moderate to severe intellectual disability, and a distinctive facial appearance and caused rearrangement of chromosome 8 that results in a deletion of a piece of the short (p) arm and a duplication of a piece of the long (q) arm
Sandal Gap: widely spaced gap between the first toe (the great toe) and the second toe
Sanger sequencing: method of DNA sequencing based on the selective incorporation of chain-terminating dideoxynucleotides by DNA polymerase during in vitro DNA replication
Schizophrenia: long-term mental disorder of a type involving a breakdown in the relation between thought, emotion, and behavior, leading to faulty perception, inappropriate actions and feelings, a sense of mental fragmentation, and withdrawal from reality and personal relationships into fantasy and delusion
Sclerocornea: scarring of the cornea of the eye
Scoliosis: side-to-side curvature of the spine
Septate: having or partitioned by a septum or septa
Septum pellucidum: thin, triangular, vertical membrane separating the anterior horns of the left and right lateral ventricles of the brain
Sex chromosomes: chromosomes carrying the genes responsible for determining the sex of offspring, in humans the sex chromosomes are designated X and Y; females have 2 X chromosomes and males have one X and one Y chromosome
Sex-linked: disorders that are inherited through one of the sex chromosomes, X or Y
Single nucleotide polymorphism (SNP): differences at the individual nucleotide level among individuals
Sinus: cavity in the body
Situs inversus: condition in which the heart and other organs of the body are transposed through the sagittal plane to lie on the opposite side from the usual.
Skin tag: benign growths thought to be caused by skin friction
Smith–Magenis syndrome: complex developmental disorder characterized by distinctive facial features, skeletal malformations, varying degrees of intellectual impairment, speech and motor delays, sleep disturbances, and self-injurious or attention-seeking behaviors; caused by deletion of the RAI1 gene located on the short arm of chromosome 17 within the 17p11.2 region
Somatic mutation: change in the genetic structure that is neither inherited nor passed to offspring; can occur in any of the cells of the body except the germ cells (sperm and egg)
Sotos Syndrome: overgrowth disorder, characterized by excessive growth prenatally and postnatally, a large elongated head, distinctive facial configuration, and a non-progressive neurological disorder with intellectual disability; may show advanced bone age; caused by mutation in the NSD1 gene located on the long arm of chromosome 5 at 5q35.2-q35.3
Spasticity: state of increased tone of a muscle (and an increase in the deep tendon reflexes
Spherocytosis: disorder of the red blood cell membrane clinically characterized by anemia, jaundice (yellowing) and splenomegaly (enlargement of the spleen)
Spina bifida: major birth defect and a type of neural tube defect that involves an opening in the vertebral column caused by the failure of the neural tube to close properly during embryonic development
Split foot (ectrodactyly): longitudinal deficiency of a digital ray of the foot except rays 1 or 5
Split hand (ectrodactyly): longitudinal deficiency of a digital ray of the hand except rays 1 or 5
Stem cell: an undifferentiated cell that retains the ability to give rise to other, more specialized cells
Stenosis: abnormal narrowing or contraction of a duct or canal
Sternal tag: pedunculated skin and subcutaneous tissue originating from the sternal area
Stop codon: Any of three codons, UAA, UAG, or UGA, that signal the termination of the synthesis of a protein
Strabismus: disorder of vision due to a deviation from normal orientation of one or both eyes so that both cannot be directed at the same object at the same time; squint; crossed eyes
Subluxation: incomplete or partial dislocation
Subtelomeric: below the telomere, the end of a chromosome
Sulci: shallow furrows on the surface of the brain separating adjacent convolutions
Supernumerary: present in excess of the normal or requisite number, in particular
Supravalvular aortic stenosis: fixed form of congenital left ventricular outflow tract (LVOT) obstruction that occurs as a localized or diffuse narrowing of the ascending aorta beyond the superior margin of the sinuses of Valsalva
Symblepharon: adhesion between an eyelid and the eyeball
Syndactyly: condition in which fingers or toes are joined together; can involve the bones or just the skin
Syndactyly, cutaneous fingers: soft tissue continuity in the A/P axis between two fingers that extends distally to at least the level of the proximal interphalangeal joints
Syndactyly, cutaneous toes: soft tissue continuity in the A/P axis between adjacent foot digits that involves at least half of the P/D length of one of the two involved digits
Syndrome: group of three or more characteristics and/or symptoms that occur together in a recognizable pattern and cause a particular disease state or condition
Synophrys: meeting of the medial eyebrows in the midline
Synostosis: union or fusion of adjacent bones by the growth of bony substance

T: the abbreviation for thymine, a pyrimidine nitrogenous base found in the structure of DNA; in RNA, thymine is replaced by uracil
Tandem repeats: sequence of two or more DNA base pairs that is repeated in such a way that the repeats lie adjacent to each other on the chromosome
Telecanthus: abnormally increased distance between the medial canthi (corners of the eye) of the eyelids
Telocentric chromosome: a chromosome with a centromere at the end; also called an acrocentric chromosome;
Telomere: the ends of a eukaryotic chromosome
Teratoma: type of germ cell tumor that may contain several different types of tissue and sometimes mature elements such as hair, muscle, and bone
Terminal deletion: deletion that occurs towards the end of a chromosome
Tetralogy of Fallot: the combination of four different heart defects: ventricular septal defect (VSD), pulmonary stenosis (obstructed outflow of blood from the right ventricle to the lungs), dextroposition or overriding aorta (displaced aorta, causing blood to flow into the aorta from both right and left ventricles); and right ventricular hypertrophy (abnormal enlargement of the right ventricle)
Tetrasomy: four copies of a particular chromosome present in a cell, resulting in 48 chromosomes in the cell instead of the usual 46
Thenar region: fleshy mass of the outer side of the palm of the hand
Thrombocytopenia: low blood platelet count
Thumb, absent: absence of both phalanges of a thumb and the associated soft tissues
Thumb, adducted: in the resting position, the tip of the thumb is on, or near, the palm, close to the base of the fourth or fifth finger
Thumb, triphalangeal: thumb with three phalanges in a single, proximodistal axis
Thymine (T): pyrimidine nitrogenous base found in the structure of DNA; in RNA, thymine is replaced by uracil
Tongue, bifid: tongue with a median apical indentation or fork; may be associated with ankyloglossia
Tongue, large: increased length and width of the tongue; leads to protrusion of the tongue
Tooth, natal: erupted tooth or teeth at birth
Tooth, supernumerary: extra tooth or teeth
Townes–Brocks syndrome: autosomal dominant genetic disorder characterized by absence of the anal opening (imperforate anus), abnormal ears associated with hearing impairment and thumb malformations; abnormalities in the feet, heart and kidneys also occur frequently. Townes-Brocks syndrome is associated with a mutation in the SALL1 gene located on the long arm of chromosome 16 at 16q12.1
Tracheaesophageal fistula: birth defect in which the trachea is connected to the esophagus
Trait: phenotypic characteristic that can be inherited
Transcription: cellular process by which genetic information in the form of a gene in a DNA molecule is converted into the form of a messenger RNA (mRNA) molecule
Transfer RNA (tRNA): type of RNA molecule necessary for translation to occur properly; provides the basis of the genetic code, in which codons in a messenger RNA (mRNA) molecule are used to direct the sequence of amino acids in a polypeptide; contains a binding site for a particular amino acid and a region complementary to a messenger RNA (mRNA) codon (an anticodon)
Transition mutation: DNA mutation in which one pyrimidine (cytosine or thymine) takes the place of another, or a purine (adenine or guanine) takes the place of another.
Translation: cellular process by which genetic information in the form of a messenger RNA (mRNA) molecule is converted into the amino acid sequence of a protein, using ribosomes and RNA molecules as accessory molecules.
Translocation: movement of a chromosome segment to a nonhomologous chromosome as a result of an error in recombination
Transposition of great arteries: birth defect in which the two major vessels that carry blood away from the heart the aorta and the pulmonary artery are switched (transposed)
Transverse limb reduction: partial or complete absence of a part of one or more limbs in the absence of other abnormalities
Tricho–rhino–phalangeal syndrome: genetic disorder consisting of fine and sparse scalp hair, thin nails, pear-shaped broad nose, and cone-shaped epiphyses of the middle phalanges of some fingers and toes
Trigonocephaly: Wedge-shaped or triangular head, with the apex of the triangle at the midline of the forehead and the base of the triangle at the occiput, assessed from above
Triploid: possessing three complete sets of chromosomes
Trisomy: genetic condition involving one chromosome of a homologous chromosome pair that has been duplicated in some way, giving rise to a 2N + 1 genotype and causing serious phenotypic abnormalities; well-known trisomies are Trisomy 13, 18, and 21
Trisomy 13: see Patau syndrome
Trisomy 18: see Edwards syndrome
Trisomy 21: see Downs syndrome
Truncus arteriosus: congenital heart defect in which there is a single main blood vessel carrying blood away from the heart which then branches into other blood vessels; with no pulmonary artery to carry blood to the lungs, and aorta blood from both ventricles of the heart is mixed, resulting in some oxygen-rich blood travelling back to the lungs and some oxygen-poor blood travelling to the rest of the body
Tuberous sclerosis (TS): genetic multisystem disorder characterized by episodes of uncontrolled electrical activity in the brain (seizures), intellectual impairment, distinctive skin abnormalities (lesions), and benign (noncancerous), tumor-like nodules (hamartomas) that appear in the brain, certain regions of the eyes (e.g., retinas), the heart, the kidneys, the lungs, or other tissues or organs; caused by mutations of either the TSC1 gene located on the long arm (q) of chromosome 9 at 9q34 or the TSC2 gene located on the short arm (p) of chromosome 16 at 16p13.
Turner syndrome: a human genetic defect in which a female has only forty-five chromosomes and is lacking a second X sex chromosome; abnormalities such as sterility and anatomical defects are common
Turricephaly: tall head relative to width and length; head length and width are reduced compared to normal age-related standards

Unbalanced translocation: translocation of chromosomes resulting in the gain or loss of genetic material that can cause problems in health, growth or development
Uniparental disomy: both chromosomes coming from one parent rather than one from each; may be maternal or paternal in origin
Univentricular heart: anomaly in which all blood flows through one ventricle or in which the arterioventricular valves are committed to empty into only one chamber in the ventricular mass
Uracil (U): pyrimidine nitrogenous base found in the structure of RNA; in DNA, uracil is replaced by thymine

VACTERL–association: acronym for association of seven birth defects—vertebral abnormalities (V), anal atresia (A), cardiac (heart) defects (C), tracheal-esophageal abnormalities (TE), renal/kidney and radial abnormalities (R), limb abnormalities (L) and single umbilical artery (S); cause unknown
Variant: something that differs in some characteristic from the class to which it belongs
Velopharyngeal insufficiency: improper closing of the velopharyngeal sphincter (soft palate muscle) during speech characterized by an acute nasal quality of the voice
Ventricular septal defect: hole or defect in the muscular wall of the heart that splits the lower chamber into two ventricles; small holes can close spontaneously; large holes can lead to progressive heart failure and require surgery
Vermilion, lower lip, everted: inner aspect of the lower lip vermilion (normally apposing the teeth) visible in a frontal view
Vermilion, upper lip, tented: triangular appearance of the oral aperture with the apex in the midpoint of the upper vermilion and the lower vermilion forming the base
Vesico–ureteral reflux: abnormal flow of urine from the bladder to the upper urinary tract
Visceral: referring to the viscera, the internal organs of the body, specifically those within the chest (as the heart or lungs) or abdomen (as the liver, pancreas or intestines)

Waardenburg syndrome: genetic disorder with evident genetic anomolies, including distinctive facial abnormalities; unusually diminished coloration (pigmentation) of the hair, the skin, and/or the iris of both eyes (irides); and/or congenital deafness; four types are known and thought to be caused by mutations in EDN3, EDNRB, MITF, PAX3 and SOX10 genes; gene responsible for WS1 is the "PAX3" gene located on the long arm (q) of chromosome 2 at 2q35
WAGR syndrome: 11p deletion syndrome; acronym for Wilms' Tumor (W), aniridia (A), genitourinary abnormalities (G), mental retardation/intellectual disability (R)
Webbed neck: neck that appears to be unusually broad because of bilateral folds of skin extending from the clavicles to the head but containing no muscles, bones, or other structures
Whole exome sequencing: mode of identifying those variants found in the coding region of genes which affect protein function
Whole-genome sequencing: laboratory process that determines the complete DNA sequence of an organism's genome at a single time
Wild-type: interacting with more than one messenger RNA (mRNA) codon by virtue of the inherent flexibility present in the third base of the anticodon; refers to the phenotype of the typical form of a species as it occurs in nature
Williams syndrome: autosomal dominant chromosomal disorder characterized by intellectual disability, unique personality characteristics, unusual facial features, cardiovascular disease .and high level of calcium in blood (hypercalcemia) and urine (hypercalciuria); caused by deletion on chromosome 7 at band 7q11 and includes the ELN (elastic) gene
Wilms’ tumor: a type of malignant tumor of the kidney that occurs in young children
Wolf–Hirschhorn syndrome (4p-syndrome): chromosomal disorder characterized by distinctive facial appearance, delayed growth and development, intellectual disability, and seizures and caused by deletions near the end of the short (p) arm of chromosome 4 which include the genes WHSC1, LETM1, and MSX1
Wolff–Parkinson–White syndrome: abnormality in the electrical functioning of the heart which may cause rapid heart rates; affects the electrical signal between the atria and ventricles

X linkage: a genetic phenomenon involving a gene located on the X chromosome; the typical pattern of X linkage involves recessive alleles, such as that for hemophilia, which exert their effects when passed from mother to son and are more likely to be exhibited by males than females
X-inactivation: phenomenon in a female by which one X chromosome (either the maternally or paternally derived X) is randomly inactivated in an early embryonic cell, with fixed inactivation of that same X in all cells descended from that cell.

Y linkage: a genetic phenomenon involving a gene located on the Y chromosome; as a result, such a condition can only be passed from father to son