Most cats, including domestic breeds, have 19 pairs of chromosomes for a total of 38. Some types of cat in South America, however, have only 36 chromosomes, including ocelots, kotkots and margays. Dogs have more than double the number of chromosomes with 39 pairs, while humans have 23 pairs.
Researchers at U.S. National Cancer Institute discovered that the way the genes are organized in the sex chromosomes, the X and Y chromosomes, of cats very closely resembles that of humans. This may explain why cats and humans share many hereditary medical problems, such as diabetes and hemophilia. It may also explain why viruses tend to behave in the same way in both humans and cats.