Single-nucleotide variation (SNV) When a single base pair (A, C, T, G) is observed to vary in a DNA sequence. This variant may be private to an individual or family. For biallelic SNVs (i.e., an SNV that exists in two possible states), an individual can carry three possible allelic combinations or genotypes on their two chromosomes (e.g., for a single base pair position that could be A or G, the three possible genotypes are AA, AG, or GG).