title: Protease inhibitor (Pi) locus, fertility and twinning.
abstract: 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.
authors.affiliation: Department of Psychonomics, Free University, Amsterdam, The Netherlands.
authors.forename: D I
authors.initials: DI
authors.lastname: Boomsma
authors.forename: R R
authors.initials: RR
authors.lastname: Frants
authors.forename: R A
authors.initials: RA
authors.lastname: Bank
authors.forename: N G
authors.initials: NG
authors.lastname: Martin
mesh_terms: D000483:Alleles; D005191:Family Characteristics; D005260:Female; D005298:Fertility / Q000235:genetics*; D005787:Gene Frequency / Q000235:genetics; D006801:Humans; D008297:Male; D008875:Middle Aged; D010641:Phenotype; D011110:Polymorphism, Genetic / Q000235:genetics; D011247:Pregnancy; D011272:Pregnancy, Multiple / Q000235:genetics*; D014427:Twins / Q000235:genetics*; D014429:Twins, Dizygotic / Q000235:genetics; D014430:Twins, Monozygotic / Q000235:genetics; D000515:alpha 1-Antitrypsin / Q000235:genetics*
chemical_list: D000515:alpha 1-Antitrypsin