Dogs have 39 pairs of chromosomes, for a total of 78. This works out to 76 autosomes and two sex chromosomes. As with other mammals, the karyotype of male dogs is XY whereas females' karyotype is XX.

Although dogs have 39 pairs of chromosomes (compared to 23 pairs for humans), dogs have fewer genes overall. Researchers sequencing the canine genome have identified around 19,000 dog genes compared to the 25,000 or more genes in the human genome. Despite having 6,000 fewer genes than humans, dogs exhibit a diverse range of phenotypes, from animals as large as a Great Dane or St. Bernard to toy dogs weighing no more than a few pounds. Geneticists think the canine genome may be especially prone to gene duplications and chromosomal rearrangements compared to the genomes of humans and other mammals.