WASHINGTON (AFP) -- U.S. scientists have identified two genes responsible for macular degeneration, the gradual deterioration of eyesight in the elderly that can lead to blindness, a study showed this week.

The research published in the Journal of the American Medical Association also showed that smoking and being overweight carry a strong risk of the condition, in which the central part of the eye's retina degenerates.

It was carried out on 1,466 people suffering from the condition, aged 55 to 80 in several U.S. hospitals over 11 years, from 1990 to 2001.

Subsequent study of the gene types in those surveyed revealed that mutations in two particular genes -- identified as CFHY402H and LOC387715 -- were linked to a worsening of the condition.

Smoking and being overweight increased by 19 times the risk of aggravated macular degeneration, said the study's main author, Johanna Seddon of the Tufts-New England Medical Center in Boston.