Theoretical Models in Twin Research

The study of a twin population, nonclassified by zygosity, allows to determine the relationships existing between the distribution of phenotypes and of the genotypes which determine the latter. It also allows to determine the frequency of the genotypes themselves and of their probability of manifestation in the general population from which the observed twin pairs are drawn. To this purpose, it is sufficient to draw from the twin material the information concerning within-pair combinations of the phenotypes under consideration and of sexes. For simplicity sake, we shall consider the theoretical model of a trait with only two phenotypes determined, respectively, by the presence or absence of the observed trait. to allow the determination of gene frequency and penetrance in the general population, by means of the analysis of distribution of phenotypic combina tions in the twin pairs.