The white horse has been a figure of beauty and prestige since ancient times, kept by the Persian emperor Xerxes - as recorded in Herodotus' histories - and selectively bred by royalty and horse-lovers alike.

Now, a team from Uppsala University in Sweden has found what makes a white horse white - a mutation that causes premature hair greying and a susceptibility to melanoma.

The mutation is caused by a duplication of DNA on horse chromosome 25, which affects two neighbouring genes, STX17 and NR4A3. Both of these genes are over-expressed in melanomas, a very high incidence of which is found in white horses.

The researchers, led by Leif Andersson, propose that the mutation stimulates growth of melanocytes, leading to a premature loss of the melanocyte stem cells needed for hair pigmentation. They also propose that the mutation promotes an expansion of some of the melanocytes, causing skin pigmentation.

White horses are born coloured - black, brown or chestnut - but start to grey in their first year and become completely white by six to eight years. The skin, on the other hand, remains pigmented, with many greys developing a vitiligo-like appearance and speckling.

They also believe all grey horses inherited the mutated allele from a common ancestor - all 700 horses studied, from eight different breeds, carried the allele. The popularity of the colour led to selective breeding and it is now estimated that one horse in 10 carries the mutation.

The findings were published online today in Nature Genetics [DOI: 10.1038/ng.185].