GENETICS Going gray

Q:In my 70 years, I have seen many a gray horse, both dappled or flea-bitten, turn totally “white” with age. But now I have two---a 21-year-old Quarter Horse and an 18-year-old Quarter Horse-draft mix---who over the last few years have been becoming more and more fleabitten. Even the one that was dappled is now going flea-bitten. Could this be due to a dietary issue? Mary Lewis Callicoon Center, New York

A:Gray horses are caused by an interesting gene. The gene is dominant and can appear in combination with any other coat color or pattern. At first, the gene seems to increase pigmentation, so that many young gray horses are quite dark. Over time, the pigment cells begin to decrease, and the horse gradually whitens. Some gray horses go through a nicely dappled stage; others less so.

However, in some horses the gray gene seems prone to revert to the original form, so that some areas of the coat regain the ability to produce pigment. This is what happens in the flea-bitten areas---these are small spots where the D. Phillip Sponenberg, DVM, PhD, is a professor of pathology and genetics at Virginia–Maryland Regional College of Veterinary Medicine. He is the author of Equine Color Genetics, and his areas of research include genetics of domesticated animals, coat color heredity, conservation of rare livestock breeds, diagnostic pathology and reproductive pathology.