Complex Genetics And Implications For Psychiatry

cotwin is also affected? If both monozygotic and dizygotic twin pairs have similar high concordance rates, a similar (shared) environment is a major factor for susceptibility. Similar low concordance rates are due to different (nonshared) environmental influences. A genetic influence is indicated if monozygotic are significantly higher than dizygotic concordance rates. If only one gene contributes in each family, we expect monozygotic twins to have a twofold higher concordance rate than dizygotic twins, since they share twice as many genes as dizygotic twins. If more than one genetic factor contributes to susceptibility in the same person (which can be two or more different genes, or, as in a recessive disorder, two alleles of the same gene), the difference between the concordance rate for monozygotic versus dizygotic twins is expected to be larger than twofold. Such a contribution of multiple genes to a given illness is often called epistatic gene interaction (Lander and Schork 1994