|MadSci Network: Genetics|
As you are probably aware, the tortoiseshell (black/orange) color combination in cats is due to inactivation of one X chromosome in each cell of female mammals. The coat color gene O/o is located on the X chromosome. Individuals with only the O allele have orange fur containing phaeomelanin, which is a reddish pigment. Ones with only the o allele have fur containing melanin, which is dark brown to black in color. (The Cat Fanciers site has a nice page about color inheritance in cats.) A heterozygous female has one X chromosome with the O allele and one carrying the o allele. Early in embryonic development, one X chromosome is inactivated (randomly) in each cell; descendant cells continue to have that chromosome inactive and the other active. In the skin of the adult, then, there are patches of cells descended from a single embryonic cell, and some patches have an O on the active X, and produce orange fur, while others have an o on the active X, and produce black fur. This color pattern is called tortoiseshell, and if white spotting (controlled by another gene that isn't sex-linked) is also present, the pattern is called calico. Males normally have only one X, so they can't be calico or tortoiseshell, just orange or black all over. In rare instances a male has two X chromosomes and a Y chromosome, and then X-inactivation takes place just as it would for a female. This produces a tortoiseshell or calico male, if he is heterozygous for the O/o gene. XXY cats are usually sterile (Centerwall and Benirschke, 1975). The XXY condition is called Klinefelter's syndrome in humans, and has similar effects, except that no skin or hair color genes appear to be located on the human X chromosome, so you don't get calico humans. You can find lots of information about Klinefelter's syndrome (mainly in humans, though, as well as on X inactivation by searching PubMed Central. Reference: Centerwall WR, Benirschke K. An animal model for the XXY Klinefelter's syndrome in man: tortoiseshell and calico male cats. Am J Vet Res. 1975 Sep;36(9):1275-80.
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