Age-related macular degeneration, a leading cause of blindness in older adults, is greatly influenced by a person's genes, according to a study of twins. "Based on what is, to our knowledge, the largest twin study of age-related macular degeneration to date ... we quantified substantial genetic influences," study author Johanna Seddon of Harvard Medical School wrote in the most recent issue of the Archives of Ophthalmology. In the study of 840 people with a twin, identical twins with macular degeneration were twice as likely than fraternal twins with the disease to suffer the same degree of blindness. Among identical twins with the disease, 55 percent had the same severity of the disease, compared to 25 percent of the fraternal twins, researchers said. Environmental factors, such as whether the victim smoked or was obese, contributed to the severity in 19 percent to 37 percent of cases. Identical twins are monozygotic, coming from the same egg that splits after fertilization and sharing the same genes. Fraternal twins are dizygotic, meaning two eggs were fertilized simultaneously. Last week, scientists reported the discovery of a variation in a single gene that could be responsible for half of all cases of age-related macular degeneration, which afflicts between 10 million and 15 million people in the United States. The gene is involved in a component of the immune system that regulates inflammation, they reported in the journal Science. Macular degeneration involves the accumulation of waste deposits in the macula, the center of the retina that is packed with cones which help in seeing color, detecting motion and making out fine detail. It erodes the center of the field of vision.