Half of all cases of an eye disease which is a leading cause of blindness are caused by a faulty gene, US scientists suggest.
AMD affects part of the retina
Age-related macular degeneration (AMD) affects 500,000 people in the UK.
The team behind the study, published in Science, say it could lead to new ways of preventing and treating AMD.
A UK charity said identifying those at risk of AMD could allow them to change their lifestyles to cut their chances of developing the disease.
But the Macular Disease Society cautioned gene therapy for AMD was likely to be many years away.
Factors which can increase a person's risk of developing the disease include smoking and obesity.
Macular disease affects the central part of the retina. It generally involves both eyes, although they may not be affected at the same time or to the same degree.
Ninety per cent of AMD cases are dry AMD, where visual cells stop functioning. This cannot be treated.
But 10% are wet AMD, where new blood vessels grow under the centre of the retina, leaking fluid and causing scar tissue to form. This destroys central vision over a period of between two months and three years.
Scientists from the US National Eye Institute, Yale University and the Rockefeller University screened members of 82 families affected by AMD, 495 other individuals with the condition and 185 unrelated people who did not have AMD.
People with a specific variant of a gene called complement factor H (CFH), found on chromosome 1, were more likely to have AMD compared with those with other versions of the gene.
The genetic mutation was present in half of those who had the disease, or who had the highest risk of developing it.
The association was particularly strong with people who had the wet form of the disease.
Margaret Pericak-Vance, one of the report's authors and director of the Duke Center for Human Genetics, said: "This gene opens the door to a whole new understanding of the factors that contribute to this disease.
"The finding may ultimately lead to new methods for identifying those at high risk for macular degeneration and suggests new pathways for drug development."
But she said, more immediately, if people at risk could be identified before symptoms developed, it might be possible to change people's lifestyles to minimise their chances of developing AMD.
'A step forward'
Andrew Webster, honorary consultant ophthalmologist at Moorfields Eye Hospital in London, said the "significance and importance" of the research finding should not be underestimated.
"It has been known for some time that the genes you inherit from your parents significantly influence your risks of losing vision due to age," he said.
"However, the size of this genetic effect, the number of genes involved and the identity of the genes has remained a mystery.
"This landmark research appears to show conclusively that the inheritance of different copies of one gene, CFH, significantly affect ones chances of losing vision due to age-related macular degeneration."
Dr Bob Thompson, president of the UK's Macular Disease Society, called the research a "step forward".
But he cautioned the "real problem is getting the gene technology to correct the fault".
"However, if you can identify people with this gene fault younger in life than we have been able to, it would be possible to make lifestyle changes to mitigate their risk," he said.