Allele:
An alternate form of a gene found at the same locus on homologous chromosomes.

Autosome:
Any chromosome other than a sex (X or Y) chromosome.

Barr body:
Inactive X chromosome seen in somatic female cells.

Carrier:
One who has a gene pair consisting of one normal and one abnormal or mutant gene. Usually, such persons are phenotypically normal.

Congenital:
Present at birth. Not necessarily genetic, often environmental.

Cytogenetics:
The study of the relationship between the microscopic appearance of chromosomes during cell division and a person's genotype and phenotype.

Deoxyribonucleic acid (DNA):
The nucleic acid of the chromosomes; it carries the genetic code.

Expressivity:
Variability of severity of a given trait or disorder, as contrasted with penetrance, which refers to presence or absence of the trait.

Genotype:
The genetic constitution.

Heterozygous:
The presence of two different alleles, one of which is normal, at a given locus on a pair of homologous chromosomes.

Homozygous:
The presence of identical genes at a given locus on a pair of homologous chromosomes.

Karyotype:
The chromosomal constitution as studied microscopically. The term is often used to refer to a photomicrograph of a person's chromosome set arranged in a standard pattern.

Locus:
The position of a gene on a chromosome.

Mosaic:
A person carrying at least two cell lines of different genotype or karyotype that developed after formation of a normal zygote.

Mutation:
Alteration of a gene, which may have a positive, indifferent, or negative effect. If no effect, the term polymorphism is usually used.

Nondisjunction:
Failure of two chromosomes to separate during meiosis or mitosis.

Penetrance:
Presence or absence of a gene effect.

Phenocopy:
An environmentally induced mimic of a genetic disorder.

Phenotype:
The entire physical, biochemical, and physiologic nature of a person, resulting from the interaction between environment and genotype.

Pleiotropism:
Ability of a gene or gene pair to produce multiple effects.

Propositus:
A family member who first draws attention to a pedigree for a given trait. Also called index case or proband.

Sporadic trait:
A trait that occurs in a single member of a kindred, there being no other affected member of the family. The trait is genetic, in contrast to nongenetic isolated cases. An alternate usage identifies all singly affected cases—genetic or environmental—as sporadic.