An organism's genotype describes its specific combination of alleles. For example, an Aa genotype is heterozygous for the A allele. An organism's phenotype describes a visible trait, such as tall height, brown eyes, or black fur. How does genotypic variation occur?

The genetic variation occurs within the individual when it carriers the different variant of a gene. Genetic variation is a result of small differences in DNA, usually by mutation. The most common type of genetic variation among individuals is single nucleotide polymorphisms (SNP). Genetic recombination also produces changes within alleles of a gene.