Domesticated horse breeds have 64 chromosomes. Przewalski's Horse, the only known remaining breed of wild horse, has 66 chromosomes. In spite of this difference, both horse breeds are able to mate together and produce fertile offspring.
Mapped in 2007 by the Horse Genome Project, the genetic material of a horse includes 2.7 billion DNA base pairs inside each chromosome. Other animals with 64 chromosomes are the chinchilla, the fennec fox and the spotted skunk. Donkeys, which have 62 chromosomes, are bred with the domesticated horse to produce mules, which have 63 chromosomes. Because of this odd number, most mules are infertile.