Protease inhibitor (Pi) locus, fertility and twinning

Hum Genet. 1992 May;89(3):329-32. doi: 10.1007/BF00220552.


In a sample of 160 Dutch twin pairs and their parents, we found that mothers of dizygotic twins had frequencies of the S and Z alleles at the protease inhibitor (Pi) locus that were 3 times higher than a control sample. Mothers of identical twins also had a higher frequency of S than controls. The S allele may thus both increase ovulation rate and enhance the success of multiple pregnancies. There was also an increased frequency of the S allele in fathers of dizygotic twins; however, this may be a secondary effect of assortative mating for family size (indicating by the number of siblings of the parents), for which a correlation of 0.2 was observed. Parents of dizygotic twins came from larger families than parents of monozygotic twins, but no effect of Pi type on family size was seen.

MeSH terms

  • Alleles
  • Family Characteristics
  • Female
  • Fertility / genetics*
  • Gene Frequency / genetics
  • Humans
  • Male
  • Middle Aged
  • Phenotype
  • Polymorphism, Genetic / genetics
  • Pregnancy
  • Pregnancy, Multiple / genetics*
  • Twins / genetics*
  • Twins, Dizygotic / genetics
  • Twins, Monozygotic / genetics
  • alpha 1-Antitrypsin / genetics*


  • alpha 1-Antitrypsin