The genetics of placental alkaline phosphatase: a possible ‘null’ allele

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Abstract
SUMMARY: Data are presented on the placental alkaline phosphatase (Pl) phenotypes of 3655 placentae, including those from 1143 dizygotic twin pairs. Discrepancies are present between the observed numbers and those expected according to Hardy‐Weinberg equilibrium, homozygotes being apparently in excess. This statistical discrepancy can be reduced to insignificance by proposing that the Pl system contains a ‘null’ allele, Pl0, in addition to the 17 alleles for which there is electrophoretic evidence. The plausibility of this suggestion is examined in the light of what is known about ‘null’ alleles in other enzyme systems and in other species.