A curious feature of cat genetics involves the determination of orange and black colors. Geneticists describe black and its various shades (black, brown, blue) as the ancestral coat color, with orange shades (red, orange, cream) developing as a mutation that was further enhanced through breeding. In cats, the gene that determines these colors is linked to x chromosomes and each x chromosome can express only one color. As a result, male cats, who have only one x chromosome, can be either orange or black, but not both (or one color mixed with white*). Females, on the other hand, have two x chromosomes and can show both coat colors on the same individual, a trait described as tortoiseshell (or calico if mixed with white). The hair color expressed by each skin cell is determined randomly, as one of the x chromosomes becomes inactivated during embryological development. Tortoiseshell cats, therefore, show a unique and permanent mixed distribution of color. Because of these genetic influences, orange cats are about three times as likely to be male as female, and virtually all tortoiseshell cats are female. For a male cat to express both orange and black colors it must either have a rare genetic variation (two x chromosomes and one y chromosome), or represent an anomaly of embryologic development, like a chimera, which occurs when two separate embryos fuse together. Only about one in three thousand tortoiseshell cats is male.
*Piebaldism, showing patches of white, has a separate genetic influence.
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