An individual's genotype is the composition, in the individual's genome, of a specific region of DNA that varies within a population. (The genome of the individual is the total collection of the DNA in a cell's chromosomes. It includes all of the individual's genes, as well as the DNA sequences that lie between them.)

Alleles differ in their DNA sequence, and may lead to production of alternative forms of the protein they encode.

A genotype could represent a single DNA nucleotide, at a specific location on a chromosome. It could also be a sequence repeated multiple times, a large duplication, or a deletion. Most variation in genotypes does not cause any difference in the proteins being produced by the cell, because genes, which code for proteins, occupy only about 2 percent of the total genome. However, when a specific genotype does affect the composition or...