Autosomes: Chromosomes that are not sex chromosomes. Autosomes make up the first 22 chromosome pairs.
Camptodactyly: Permanent flexion contracture of a finger or toe.
Clinodactyly: Permanent curving of the fifth finger, usually toward the other fingers; also seen for other fingers or toes.
Consanguinity: A genetic relationship between persons descended from a common ancestor. Consanguinity increases the likelihood of inheriting identical versions of a given gene.
Consultand: Person who seeks genetic counseling for knowledge about a disease or condition in the family.
Dysmorphism: Structural anomalies such as widely spaced eyes (hypertelorism) and large head circumference (macrocephaly).
Dysplasia: Abnormal organization of cells in tissue.
Epicanthal folds: Skinfolds covering the inner corners of the eyes. Epicanthal folds occur commonly in infants and may persist in persons of certain ancestries.
Expression: Process of converting DNA into protein.
“Hockey stick” palmar crease: Transverse flexion crease of the palm close to the fingers; widens like the end of a hockey stick and ends between the second and third fingers.
Hyperdiploid: Having more than 46 chromosomes in one cell.
Hyperphagia: Pathologically excessive appetite or eating.
Hypoplasia: Incomplete development of a tissue or organ.
Imprinting: Differential expression of a gene dependent on the sex of the parent of origin. For example, for a maternally imprinted gene there is no expression in the offspring; expression of the gene occurs from only the paternally inherited chromosome
Mental retardation: Abnormal intellectual performance, generally determined by an intelligence quotient (IQ) test. Mental retardation is divided into mild (IQ: 50 to 70) or severe (IQ: < 50). Severe mental retardation is subdivided into moderate, severe, and profound.
Methylation: Attachment of methyl groups (–CH3) to cytosine (C) bases. Methylation can regulate the transcription of genes (e.g., reducing the amount of protein generated). Hypermethylation can lead to silencing of a gene and total absence of protein production.
Mosaic or mosaicism: The presence of two or more cell lines. Mosaicism in a man with Klinefelter syndrome may consist of a cell line with a 47,XXY karyotype and another one with a normal number of chromosomes (46,XY).
Nondisjunction: Failure of a chromosome pair to separate properly (e.g., two X chromosomes into daughter cells).
Palpebral fissure length: Distance between the inner and outer corners of one eye.
Phenotype: Observed (clinical) findings in a person generated by the interactions between genes and the environment.
Philtrum: Vertical groove between the nose and upper lip; a flat or smooth philtrum can present in persons with fetal alcohol spectrum disorders.
Predictive genetic testing: Determination of genetic variation in an asymptomatic person to ascertain whether the probability for a given disease or condition is greater than the population-based average.
Premature ovarian failure: Amenorrhea before the age of 40 years.
Premutation: Trinucleotide repeat expansion within a gene to less than the number of repeats needed for the abnormal phenotype. A premutation can expand to the full mutation and cause the abnormal phenotype in the next generation. For example, most premutation carriers in the fragile X mental retardation 1 (FMR1) gene are phenotypically normal, although women with premature ovarian failure and men with fragile X–associated tremor/ataxia syndrome recently have been described.
Proband: The clinically affected member of a family who comes to medical attention first.
Ptosis: Drooping of the upper eyelids.
“Railroad track” ears: The top part (curve) of the outer ear is underdeveloped, folded over, and parallel to the curve beneath it, giving the appearance of a railroad track.
Sex chromosomes: X and Y chromosomes (in contrast with autosomal chromosome pairs 1 through 22).
Trinucleotide (or triplet) repeat expansion: Expansion of a segment of DNA within a gene that contains a repeat of three nucleotides (triplet repeats). This increase of triplet repeats happens as the gene is passed on from one generation to another. It can lead to abnormal gene expression and function. Triplet repeat diseases include fragile X syndrome, Huntington disease, myotonic dystrophy, and Friedreich ataxia.
Uniparental disomy: Abnormal presence of both copies of a chromosome (or part of a chromosome) inherited from one parent.
Vermilion border: Edge of the lip where it meets the skin of the face.
X-linked inherited disorder: Inheritance of a gene that is located on the X chromosome. X-linked recessive inheritance of a disease typically affects males and not females (e.g., hemophilia A). There is no male-to-male transmission (i.e., an affected father will have only unaffected sons). All daughters of an affected father are phenotypically unaffected but are carriers for the gene mutation. On average, one half of sons born to carrier mothers are affected.