LONDON (Reuters) - Having genes that give you red hair, pale skin and freckles increases your risk of developing skin cancer as much as an extra 21 years' exposure to the sun, researchers said on Tuesday.

Their study found gene variants that produce red hair and freckly, fair skin were linked to a higher number of mutations that lead to skin cancers. The researchers said even people with one copy of the crucial MC1R gene - who may be fair-skinned but not have red hair - have a higher risk.

"It has been known for a while that a person with red hair has an increased likelihood of developing skin cancer, but this is the first time that the gene has been proven to be associated with skin cancers with more mutations," said David Adams, who co-led the study at Britain's Wellcome Trust Sanger Institute.

"Unexpectedly, we also showed that people with only a single copy of the gene variant still have a much higher number of tumor mutations than the rest of the population."

Redheads make up between 1 and 2 percent of the world's population, but about 6 percent in Britain. They have two copies of a variant of the MC1R gene which affects the type of melanin pigment they produce, leading to red hair, freckles, pale skin and a strong tendency to burn in the sun.

Exposure to ultraviolet light from either the sun or sunbeds causes damage to DNA and scientists think the type of skin pigment linked to redheads may allow more UV to reach the DNA.

In this latest study, the researchers found that while this may be one factor in the damage, there are also others linked to the crucial MC1R gene.

The team, whose work was published in the journal Nature Communications, analyzed data from of tumor DNA sequences collected from more than 400 people with cancer. They found an average of 42 percent more sun-associated mutations in tumors from people carrying the MC1R gene variant.

The research showed the MC1R gene variation not only increased the number of spontaneous mutations caused by sunlight, but also raised the level of other mutations in the tumors.

This suggests, the researchers said, that there are biological processes in the way cancer develops in people with MC1R variation that are not only related to ultraviolet light.

"This ... explains why red-haired people have to be so careful about covering up in strong sun," said Julie Sharp of the charity Cancer Research UK, which co-funded the research.

"It also underlines that it isn't just people with red hair who need to protect themselves from too much sun."

(Reporting by Kate Kelland; editing by Andrew Roche)