Genetic contribution to DZ twinning.

The genetic contribution to dizygotic (DZ) twinning was investigated using 6,596 twin pairs from the Australian Twin Registry who provided information on other twins in their families. Responses were classified by the zygosity (DZ; monozygotic [MZ]) of the proband twins and by the relationship and zygosity of related twins. MZ probands and MZ twins reported by DZ probands were used as controls and assumed to be independent of any genetic influence. Significantly higher proportions of DZ twins were found in the families of DZ probands compared to the families of MZ probands for the following relationships: sibs of probands, proband mothers, offspring of sisters of proband mothers, and offspring of female probands (P < 0.001 in each case). The latter 2 relationships were used to estimate risk ratios of 1.7 for sisters of mothers of DZ twins, and 2.5 for offspring of female DZ twins. No greater tendency to DZ twinning in close relatives was found in mothers who bore DZ twins at a younger age than at an older age.