About ten million people in the United States have signs of AMD, two million with decreased vision related to advanced AMD and eight million with earlier stages of the disease, according to background information in the article. Despite the public health impact of AMD, however, the relative roles of genes and environment in the development of AMD remain unclear. The authors state that classic twin studies, which compare the occurrence of a trait or disease in monozygotic (MZ, commonly called identical) twins versus dizygotic (DZ, commonly called fraternal) twins provide one of the most powerful methods for determining heritability (the relative contribution of genes versus environment).
Johanna M. Seddon, M.D., Sc.M., of Harvard Medical School, Boston, and colleagues surveyed 840 male twins from the National Academy of Sciences-National Research Council World War II Veteran Twin Registry born between 1917 and 1927. Two hundred and ten MZ and 181 DZ complete twin pairs and 58 singletons, whose twin were no longer living or were unavailable for the study, completed a survey and underwent an examination for AMD. The results were then statistically analyzed to determine the relative roles of genes, common shared environment and specific (unique to the individual) environment.
Of the 840 twins, 331 had no signs of macular degeneration, 241 had early signs, 162 had intermediate AMD and 106 had advanced AMD, the researchers found. There were no differences in the prevalence rates of AMD in MZ versus DZ twins. There were differences in whether the severity of AMD was the same for both twins depending on whether the twins were identical or fraternal, however. "Among pa
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