A significant excess of the alpha 1-antitrypsin (protease inhibitor) PiS allele has been found in 147 pairs of dizygotic (DZ) twins, but frequencies in 170 pairs of monozygotic (MZ) twins do not differ from those in a sample of 1007 blood donors. In 51 mothers of DZ twins the frequency of the PiS allele was double that in the same sample of donors, but there was no corresponding increase in the fathers of DZ twins nor in the parents of MZ twins. In an independent sample of 66 mothers of twins of unknown zygosity, there was also a significant excess of PiMPiS and PiMPiZ phenotypes, and this was particularly marked in the subsample of mothers of opposite-sex twin pairs. We speculate that lowered protease inhibitor levels in women carrying the PiS allele may enhance sperm migration, increase the probability of multiple ovulation, or both.
