We present the results of a noncentral χ2 test applied to 63 like-sexed twin pairs.

We have separated the pairs into two groups: 30 serologically identical and 33 serologically nonidentical pairs for nine sera of the blood groups ABO, MN, and Rh.

The probabilities of monozygosity of the serologically identical pairs, calculated by the method of Maynard-Smith and Penrose, range from 0.83 to 0.92 — rather small values. With the help of Belgian frequencies of DZ and MZ twins, we could prove that there are about 50% chances of having less than two DZ pairs amongst the 30 serologically identicals.

Consequently, our first group with 30 pairs is mostly MZ (less than 7 DZ pairs in 96% of the cases); while our second group, with 33 pairs, is completely DZ.

We did not try to extract out of the 30 serologically identicals, such pairs that might “seem” DZ. For instance, we have kept in our first group two twins — one with blue and one with brown eyes — because the genetics of eye colour is not yet enough well-known to be safely used in twin-diagnosis; and also in memory of Madame de Sévigné, who had one brown and one blue eye. Imperfect as the composition of our sample is, we have avoided at least one mistake: adding, to surely DZ pairs, twins who are “seemingly” DZ, but not proven to be so.