They’re rare, they’re iconic, they’re the quintessential symbol of the horse. And now their genetics are better understood.

Science Daily recently announced that an international team of researchers at Uppsala University in Sweden has identified the genetic mutation that causes true white horses. The gene can be traced back to a common ancestor that lived thousands of years ago.

The study also has implications in medical research since white horses are also at risk for melanoma. About 75 percent of grey horses older than 15 years of age have a benign form of melanoma that in some cases develops into a malignant melanoma.

A grey horse is born black, brown or chestnut. By six to eight years of age the horses have turned grey, but the skin remains pigmented. The process resembles greying in humans but the process is faster in horses.

"It is a fascinating thought that once upon a time a horse was born that turned grey and subsequently white and the people that observed it were so fascinated by its spectacular appearance that they used the horse for breeding so that the mutation could be transmitted from generation to generation," says Leif Andersson who led the study. Today about one horse in ten carries the mutation for greying with age, writes Andersson.

The researchers note that the grey mutation stimulates growth of melanocytes, which leads to a premature loss of the melanocyte stem cells needed for hair pigmentation; whereas the mutation promotes an expansion of some of the melanocytes causing skin pigmentation, says Leif Andersson.

Researchers found that the grey mutation does not change any protein structure but it affects the genetic regulation of two genes. The white horses carry an extra copy of a DNA segment located in one of these genes.

The paper was published on July 20 on the website of Nature Genetics.