ABSTRACT: Twin studies can serve as an important tool in the identification of the factors that contribute to risk for complex disease in a number of ways. They provide an efficient means for determining whether or not genetic factors are involved in the development of a complex trait or in risk for a complex disorder. By comparing concordance in monozygotic twin pairs, who are genetically identical, versus dizygotic twin pairs, whose members have, on average, only half of their genes in common, it is possible quantify the degree to which genetic factors contribute disease risk. Discordant monozygotic twin pairs can be used to screen for environmental factors that contribute to disease risk since pair members differ only with regard to the environments to which each has been exposed. Co-twin control studies involving monozygotic twin pairs are particularly efficient since cases and controls are matched for genotype in addition to age, sex, race and other demographic factors. Twin studies utilizing information on type of twin placentation can also be used to assess the importance of prenatal factors in determining risk for the development of disease in adulthood. Studies of this type could also lead to important insights about the degree to which epigenetic factors that are operative prenatally may contribute to risk of disease in adulthood. The various ways in which the twin design can be utilized in the study of complex will be discussed as well as ways in which the twin samples needed to carry out studies of this type can be ascertained.
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