White horses, such as racing’s Desert Orchid or the Lone Ranger’s Silver, are actually mutants whose defective DNA carries a gene that accelerates ageing and rapidly turns their coats grey, scientist have discovered.

Such horses would probably never have survived in the wild but for one particular white horse, born thousands of years ago, which so caught the eye of ancient humans that they protected it and did their best to breed more, according to a new study.

They were so successful that the same horse became the ancestor of almost all white horses born since. It means that Silver and Desert Orchid, which won the Cheltenham Gold Cup in 1989 and the King George IV Chase four times, were probably related.

The key finding of the researchers, at Uppsala University in Sweden, is that almost all white horses seem to carry an identical gene, implying that it originated in a single common ancestor.

White horses are unlikely to have survived in the wild. The white colouring makes them easy prey for predators, while the gene sharply raises the risk of such horses getting skin cancer. This implies that humans probably intervened to make sure they flourished.

“It is a fascinating thought that once upon a time a horse was born that turned grey and then white and the people that observed it were so fascinated that they used the horse for breeding so that the mutation could be transmitted from generation to generation,” said Leif Andersson, who led the study. The research will be published in Nature Genetics today.

Today about one horse in 10 carries the mutation, dubbed the “greying with age” gene. Such horses are brown, chestnut or black when they are born but their coats turn white within about six years.

They are distinct, however, from the rarer albino horses, which are white at birth.

Samantha Brooks, a geneticist and equine expert at Cornell University, New York, said the mutation in the “greying with age” gene meant that the pigment cells – or melanocytes – in the hair follicles in effect “dried up” early in life. The hairs keep growing but without any pigment they become white.

The absence of the pigment means the skin is less protected from sunlight and so is at greater risk of skin cancers.

“About 75% of grey horses aged over 15 years have a benign form of melanoma that may develop into a malignant melanoma,” said Andersson.

The discovery could shed light on ageing and cancer development in humans, too. White horses appear to be going through an ultrafast version of what happens in people.

Historians believe wild horses were first tamed by humans about 10,000 years ago on the steppes of central Asia. It was probably there that the first white horse was born. Since then, white horses have become associated with legend and kingship. Pegasus, the winged horse of Greek myth, is usually depicted as a grey.

King Arthur is said to have ridden a white horse, as is William the Conqueror at the battle of Hastings in 1066.

“They have this mythical image of purity and sanctity about them,” said Brooks. “They have this innocent trait about them.”

Sonya Webster, who keeps a grey mare and a stallion at End House stud farm near Clitheroe, Lancashire, said she liked them because they stood out from the crowd. There were, however, drawbacks: “They are difficult to keep clean and more likely to get sunburnt.”

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