London, July 21 (IANS) An identical mutation that explains why some horses are white can be traced back to a common ancestor living thousands of years ago. The study by Uppsala University Sweden that proved this has a bearing on medical researches as the mutation also enhances the risk for melanoma.

The bulk of white horses carry the dominant mutation ‘greying with age’. A ‘grey’ horse is born black, brown or chestnut but the greying process starts already during the first year.

Horses are completely white by six to eight years but the skin remains pigmented. Thus, the process resembles greying in humans but it is accelerated in these horses.

The research demonstrated that all ‘grey’ horses carry exactly the same mutation probably inherited from a common ancestor thousands of years ago.

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

However, the mutation promotes an expansion of some of the melanocytes causing skin pigmentation, said Leif Andersson, who led the study.

The white horse has been greatly valued as documented by the rich collection of stories and paintings featuring them. About 75 percent ‘grey’ horses over 15 years have a benign form of melanoma that in some cases develops into a malignant melanoma.

The study also gives new insight in a molecular pathway that may lead to tumour development.

The paper was published on the website of Nature Genetics.