Genetic Studies:Twin Studies

Abstract

The most general method for deciding whether, and to what extent, interindividual variation in a certain phenotype is caused by genetic variation is the study of monozygotic (MZ) twins - their similarities and differences. MZ twins originate from division of one zygote, in a very early state of embryonic development. Therefore they must as a rule be identical genetically; any phenotypic differences must largely be caused by environmental differences. Here, environment is defined in the widest possible sense: anything that is not fixed genetically. However, mutations in somatic cells outside the germ line do occur; they may lead, for example, to tumor diseases or to differences of antibodies. Mitochrondria may be distributed unevenly between the two embryos. Therefore genetic differences in such traits may occur even in MZ twins. However, these are relatively unimportant exceptions from the rule that MZ twins are identical genetically: to recognize whether and to what degree a characteristic is determined genetically and to what degree its interindividual variation is modified by environmental effects, the degree of similarity between MZ twins is an important criterion. Since dizygotic (DZ) twins are thought to be affected largely by the same environmental differences as MZ twins but to have only one half of their genes in common by decent, they are used as suitable controls (for details of the twin method,see Vogel and Motulsky 1996).