Stockholm - A mutation is believed to be the reason why some horses are white including the famous Lippizaners, according to a new study by Swedish researchers.

The mutation was traced back thousands of years to a common ancestor and was identified by researchers at Uppsala University who published their results in the online edition of Nature Genetics.

The study suggested that a large majority of white horses carry the dominant mutation causing them to grey with age.

While a grey horse is born black, brown or chestnut in colour, the greying process starts as early as the first year and the horses are normally completely white by age six to eight years even though the skin remains pigmented.

The process resembles that of greying in humans, but is ultra fast in these horses, the researchers said.

"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," said Professor Leif Andersson of the Department of Medical Biochemistry and Microbiology, who led the study.

About one in 10 horses in carries the mutation for greying with age.

The researchers also said that the mutation indicated a predisposition for the development of melanoma that can lead to skin cancer, and could offer new insight into how tumours develop.

About 75 per cent of grey horses older than 15 have a benign form of melanoma that in some cases becomes malignant. (dpa)