Genotypes

Genotype refers to the genetic makeup of an individual—the specific alleles it carries at one or more loci. In practice the term is used for the two alleles present at a given gene in a diploid organism or for the complete set of alleles across the genome. The genotype of a gene with two alleles, A and a, can be AA, Aa, or aa. The first is homozygous for the A allele; the second is heterozygous; the third is homozygous for the a allele. An organism has two copies of each chromosome, so it carries two alleles for most genes; in haploid stages there is a single allele per locus.

Genotype interacts with the environment to shape the phenotype, the observable traits. While the genotype provides the potential for certain features, environmental factors, gene expression, and regulatory networks influence the realized phenotype. Some traits are largely determined by a single gene, while others are polygenic.

In populations, genotype frequencies describe how common particular allele combinations are. In simple Mendelian cases, genotype frequencies can be predicted from allele frequencies using typical models such as Hardy-Weinberg equilibrium.

A familiar example is the ABO blood group system. Genotypes IAIA and IAi produce the A phenotype; IBIB and IBi produce B; IAIB yields AB; ii yields O.

Genotypes are determined or inferred by genetic testing, including DNA sequencing, PCR-based assays, and SNP genotyping arrays. Understanding genotype helps explain inheritance patterns and the genetic basis of traits.