Family studies on the h3 and h4 alleles. Erythrocyte phenotypes, and H and Se genotypes were determined in the members of the families of HN (A) and HA (B). The H genotypes of the members of the two families were identified by PCR-RFLP tests for detection of T721C in the H genes of HN's family members and for detection of G442T in H genes of HA's family members. The Se genotypes of the members were determined as described in a previous report.11 Square, male; circle, female; filled symbol, homozygote of the mutant H allele; half-filled symbol, heterozygote of wild-type H and mutant H alleles; open symbol, homozygote of wild-type H allele. Small closed circles and open squares in schematic alleles represent T721C in h3 allele and G442T in the h4 allele, respectively.