Inheritance of hypophosphatasia.

A system of multiple, codominant alleles (HN, HC, HI) is proposed for the inheritance of hypophosphatasia. These alleles are associated with a single autosomal locus which determines the presence or absence of hypophosphatasia among human subjects. Either HC or HI conditions hypophosphatasia whereas HN conditions the absence of this disorder. Only one genotype, HNHN, produces the absence of hypophosphatasia. Either HNHC or HNHI produces adult hypophosphatasia. Either HCHC or HCHI produces childhood hypophosphatasia. Only HIHI produces infantile (neonatal) hypophosphatasia. HI is lethal only in a homozygous state. HN occurs at a much higher frequency in human populations than either HC or HI. Any one of the hypophosphatasic genotypes is thought to exert the same degree of epistatic effect on enzyme activity within the bone/liver/kidney complex of alkaline phosphatases. These alleles and genotypes are considered to be completely penetrant but show variable expressivity within each class of hypophosphatasia.