Table I.

Definition of genetic terms

Terms	Definition
Heritability	The extent to which variation in a trait (such as cognitive performance) among members of a population is determined by inherited genetic variation.
Genome	The aggregate genetic information of an organism comprised of chromosomal and mitochondrial DNA in eukaryotes. The human genome consists of ∼3.2 billion nucleotides.
Gene	The functional unit of DNA. Generally refers to protein‐coding units but can also include non‐protein coding regulatory units.
Locus	A specific location in the genome. Can be a specific nucleotide or a larger region such a gene or cytogenetic band.
Polymorphism	Heritable trait that occurs in multiple forms in populations. Usually refers to variants in DNA but can refer to phenotypic traits. Traditionally refers to genetic variant that occurs in more than 1% of a population with mutation used for more rare variants.
Allele	One form of a polymorphic trait.
Genotype	The composition of a particular region of the genome. Generally refers to the combination of the maternal and paternal alleles of a polymorphism.
Phenotype	An observable trait that results from the combined effects of genetic and environmental factors.
Endophenotype	An endpoint for genetic association or linkage studies that is between the genome and clinically‐defined phenotypes.
Haplotype	A series of alleles that occur on the same chromosome. Usually inferred statistically from genotype data at multiple loci on a chromosome but can be determined through molecular cloning.
Linkage disequilibrium	A significant deviation from random assortment between the alleles of polymorphisms at two or more loci.
SNP	Single nucleotide polymorphism. Results from a point mutation during DNA replication which is almost always biallelic; the most common category of polymorphism.
VNTR	Variable number of tandem repeats polymorphism. Also referred to as a length polymorphism. Alleles of a VNTR differ by the number of identical or similar repeat elements.
GWAS	Genome wide association study. A genetic mapping study is one in which the entire genome is interrogated for significant association between individual polymorphisms and a target phenotype. Currently multiple genomic technologies allow screening the entire human genome at 500,000–1,000,000 SNP's using a single “chip” for each subject.