The diploid chromosome number of a domestic dog is 78.
A typical dog cell contains 38 pairs of autosomes, or non-sex chromosomes, plus two sex chromosomes. That produces 78 chromosomes in total, written as 2n = 78. Female dogs generally have two X chromosomes, while males have one X and one Y.
Diploid means that body cells carry two complete chromosome sets, with one copy of each chromosome inherited from each parent. Reproductive cells are different: eggs and sperm are haploid and contain 39 chromosomes, so fertilisation restores the diploid total of 78.
The number 78 applies across domestic dogs despite the enormous variation among breeds in size, body shape, coat, and behaviour. Chromosome count does not determine an animal’s complexity or appearance by itself; genes located on those chromosomes, along with their regulation and interactions, help produce those differences. Dogs also share the same diploid count as several related canids, including gray wolves and coyotes.