<P> Molecular genetic methods of heritability estimation have tended to produce lower estimates than classical twin studies, providing evidence that the equal environments assumption of the classic twin design may not be sound . </P> <P> A particularly powerful technique for testing the twin method was reported by Visscher et al. Instead of using twins, this group took advantage of the fact that while siblings on average share 50% of their genes, the actual gene - sharing for individual sibling pairs varies around this value, essentially creating a continuum of genetic similarity or "twinness" within families . Estimates of heritability based on direct estimates of gene sharing confirm those from the twin method, providing support for the assumptions of the method . </P> <P> Genetic factors may differ between the sexes, both in gene expression and in the range of gene × environment interactions . Fraternal opposite sex twin pairs are invaluable in explicating these effects . </P> <P> In an extreme case, a gene may only be expressed in one sex (qualitative sex limitation). More commonly, the effects of gene - alleles may depend on the sex of the individual . A gene might cause a change of 100 g in weight in males, but perhaps 150 g in females--a quantitative gene effect . Such effects are Environments may impact on the ability of genes to express themselves and may do this via sex differences . For instance genes affecting voting behavior would have no effect in females if females are excluded from the vote . More generally, the logic of sex - difference testing can extend to any defined sub-group of individuals . In cases such as these, the correlation for same and opposite sex DZ twins will differ, betraying the effect of the sex difference . </P>

The proportion of snps shared between fraternal twins is