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Heredity-Environment Interaction in the Development of Twins

As an alternative to the commonly used additive model for interpreting twin data, interactional aspects on a longitudinal twin study are presented. While experiencing the same environment MZ twins will tend to react to it similarly, while DZ twins will react differently. Interaction between genetic and environmental factors will therefore increase the difference among DZ, thus lowering the intra-class correlation with increasing age. Such lowering is not expected when interaction is absent or when genetically identical MZ's are observed. Depending on the characteristic studied and the variation allowed, interactional effects can be expected to be more or less pronounced. Examples are taken from the height and weight growth of a number of MZ and DZ twin pairs during puberty. Height seems to be a strongly genetically regulated variable showing no lowering of the intra-class correlations for the two twin categories with increasing age. Weight, on the other hand, shows an interesting sex difference. The correlation for DZ twin girls drops markedly during puberty, while this is much less pronounced for the boys. A plausible explanation for this trend would be that there are stronger interactional effects for girls than for boys as regards weight during puberty. It is difficult to explain why intra-class correlations for height for MZ and DZ twins during puberty would show a different trend than for weight without taking interactional effects into consideration. Likewise, it is hard to find a plausible interpretation of the sex difference found for weight during puberty simply on the basis of an additive model.