Figure 1. Familial DZ twinning. Five sets of DZ twins occurring in one family demonstrating probable autosomal dominant inheritance with incomplete penetrance.

Figure 2. Placentation in twins. Each developing fetus is surrounded by an inner amniotic membrane enclosing the amniotic sac and an outer chorionic membrane continuous with the placenta. The arrangement of the membranes may be useful in determining zygosity and the timing of the splitting event.

Figure 3. Familial MZ twinning. Pedigrees 1 and 2 show familial MZ twinning with apparent autosomal dominant inheritance. Pedigree 3 shows a family with both DZ and MZ twinning. MZ twins are denoted as (male twins) or (female twins).

Figure 4. Zygosity testing by analysis of hypervariable DNA. Lanes 1a and 1b contain DNA from each of a pair of DZ twins and show differences in the pattern of DNA fragments. Lanes 2a and 2b contain DNA from each of a pair of MZ twins, showing an identical pattern of DNA fragments.