Table 1.

Glossary of Genetic Terminology

Term	Definition
Biometrical genetic study	A study that seeks to quantify the influence of genes and environment on a trait or behavior, usually in the form of twin and family studies using statistical methodology such as structural equation modeling.
Polymorphic allele	A specific position (locus) in the genome that possesses more than one variant in the population. A monomorphic allele, on the other hand, is an allele that has only one observable form in the population.
Heritability	The proportion of a trait’s phenotypic variance that is due to genetic variance in the population.
Genomic research	Methods that involve the examination of a large set of genetic variants across the genome, such as genome-wide association (GWA) and genome-wide complex trait analysis (GCTA).
Phenotype	An observable characteristic or set of observable characteristics (in the case of a disease or disorder).
Gene	A single unit of genetic information that codes the amino acid sequence of a polypeptide.
Single nucleotide polymorphism (SNP)	A variation of a single base pair at a specific genomic location.
Polygenic	Influenced by a multiple genes.
Genome-wide association (GWA) study	An examination of a large number of SNPs across the genome to see if any are associated with a specific trait.
Genome-wide significant associations	A statistical test meeting a stringent significance criterion based on a multiple-testing correction that accounts for the high number of associations tested in genomic research. Typically p < 5 × 10− 8.
Mendelian	A pattern of biological inheritance following the genetic laws proposed by Gregor Mendel.
Genomic locus/loci	A specific position on a chromosome (singular: locus, plural: loci).
De novo mutation	A genetic mutation that appears in a child due to a mutation in the germ cell of a parent or in the fertilized egg.
Research domain criteria (RDoC)	A project initiated by the National Institutes of Mental Health (NIMH) that aims to examine psychiatric disorders in terms of more basic constructs, such as genes and neural circuitry.
Copy number variant (CNV)	A section of the genome that is repeated a variable number of times within the population. The number of copies can range from missing to multiple copies.
Genome-wide complex trait analysis (GCTA)	A method of estimating heritability using unrelated individuals, by directly estimating the degree of relationship for each pair of individuals in the dataset (via measured genetic variants).
Clumping	A step in constructing a traditional polygenic risk score (PRS), whereby only the most significant SNP in a specified genomic region (e.g., 500 kb) is retained for inclusion in the PRS.
Linkage disequilibrium (LD)	The non-random association of alleles at different genetic loci, insofar that alleles closer together on a chromosome are more likely to be inherited together.