Abiotrophic disease: Genetically determined disease which is not evident at birth but which becomes manifest later in life.

Acquired: Contracted after birth or in utero.

Alleles: Alternative forms of an individual gene.

Anticipation: Progressive increase in severity of autosomal dominant disease in successive generations.

Autosomes: The chromosomes (22 pairs of autosomes in humans) other than the sex chromosomes.

Chromosome: A small thread-like or rod-like structure into which the nuclear chromatin separates during mitosis. The number of chromosomes is constant for any given species (23 pairs in humans: 22 pairs of autosomes and one pair of sex chromosomes).

Codominant inheritance: Inheritance pattern in which the individuals heterozygous for the abnormality have a phenotype distinguishable from that of the homozygote.

Congenital: Existing at or before birth; not necessarily hereditary.

DNA probes: DNA fragments used to locate specific gene sequences to which they are complementary.

Dominant: Designating a gene whose phenotypic effect largely or entirely obscures that of its allele.

Expressivity: Variability of phenotype amongst genotypically identical individuals.

Familial: Pertaining to traits, either hereditary or acquired, which tend to occur in families.

Gamete (germ cell): A cell that is capable of uniting with another cell in sexual reproduction (ie, the ovum and spermatozoon).

Gene: A unit of heredity which occupies a specific locus in the chromosome which, either alone or in combination, produces a single characteristic. It is usually a single unit that is capable of self-duplication or mutation.

Genetic carrier state: A condition wherein a given hereditary characteristic is not manifest in one individual but may be genetically transmitted to the offspring of that individual.

Genotype: The hereditary constitution, or combination of genes, that characterizes a given individual or a group of genetically identical organisms.

Germ cell: See Gamete, above.

Hereditary: Transmitted from ancestor to offspring through the germ plasm.

Heterozygous: Having two members of a given hereditary factor pair that are dissimilar, ie, the two genes of an allelic pair are not the same.

Homozygous: Having two members of a given hereditary factor pair that are similar, ie, the two genes of an allelic pair are identical.

Linkage studies: Statistical analysis of frequency of association of genetic abnormalities to estimate the proximity of the gene loci. Lyon hypothesis: Inactivation of one X chromosome in each somatic cell of the female, the inactivated chromosome forming the sex chromatin (Barr) body.

Meiosis: A special type of cell division occurring during the maturation of sex cells, by which the normal diploid set of chromosomes is reduced to a single (haploid) set, two successive nuclear divisions occurring, while the chromosomes divide only once.

Mitosis: Cell division in which daughter nuclei receive identical components of the number of chromosomes characteristic of the species.

Monosomy: The existence of one chromosome of one variety, rather than the normal pair of chromosomes.

Mosaicism: The presence of cells of functionally different genetic constitution within the same individual. Normally present in the female in respect of the X chromosome (Lyon hypothesis).

Mutation: A transformation of a gene, often sudden and dramatic, with or without known cause, into a different gene occupying the same locus as the original gene on a particular chromosome; the new gene is allelic to the normal gene from which it has arisen. Penetrance: The likelihood or probability that a gene will become morphologically (phenotypically) expressed. The degree of penetrance may depend upon acquired as well as genetic factors.

Phenotype: The visible characteristics of an individual or those which are common to a group of apparently identical individuals.

Recessive: Designating a gene whose phenotypic effect is largely or entirely obscured by the effect of its allele.

Sex chromosome: The chromosome or pair of chromosomes that determines the sex of the individual. (In the human female, the sex chromosome pair is homologous, XX; in the male, heterologous, XY.)

Sex linkage: See X linkage, below.

Somatic cells: Cells incapable of reproducing the organism.

Translocation: Exchange of DNA fragments between chromosomes at the time of meiosis.

Trisomy: The existence of three chromosomes of one variety, rather than the normal pair of chromosomes.

X linkage: The pattern of inheritance of genes located on the X chromosome.

Zygote: The cell formed by the union of two gametes in sexual reproduction.