Scientists Have Finally Found the Gene That Gives Cats Orange Fur

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Most orange cats are boys, a quirk of feline genetics that also explains why almost all calicos and tortoiseshells are girls. Scientists curious about those sex differences—or perhaps just cat lovers—have spent more than 60 years unsuccessfully seeking the gene that causes orange fur and the striking patchwork of colors in calicos and tortoiseshells. Now, two teams have independently found the long-awaited mutation and discovered a protein that influences hair color in a way never seen before in any animal... Using skin samples collected from various cats, the researchers were able to hone in a mutation on the X chromosome that impacts how much of a protein a gene called Arhgap36 produces. Increasing the amount of the Arhgap36 in pigment producing cells called melanocytes activates a molecular pathway that produces a light red pigment.

"Scanning a database of 188 cat genomes, Barsh's team found every single orange, calico, and tortoiseshell cat had the exact same mutation," writes Science magazine. "The group reports the discovery this month on the preprint server bioRxiv. A separate study, also posted to bioRxiv this month, confirms these findings... They also found that skin from calico cats had more Arghap36 RNA in orange regions than in brown or black regions."
Arhgap36's inactivation pattern in calicos and tortoiseshells is typical of a gene on the X chromosome, says Carolyn Brown [a University of British Columbia geneticist who was not involved in either study], but it's unusual that a deletion mutation would make a gene more active, not less. "There is probably something special about cats." Experts are thrilled by the two studies. "It's a long-awaited gene," says Leslie Lyons, a feline geneticist at the University of Missouri. The discovery of a new molecular pathway for hair color was unexpected, she says, but she's not surprised how complex the interactions seem to be. "No gene ever stands by itself."

Lyons would like to know where and when the mutation first appeared: There is some evidence, she says, that certain mummified Egyptian cats were orange. Research into cat color has revealed all kinds of phenomena, she says, including how the environment influences gene expression. "Everything you need to know about genetics you can learn from your cat."

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