Abstract
A large survey of hemophilia A carried out with almost complete ascertainment on the island of Sardinia suggests that the variation of plasma levels of Factor VIII coagulant activity in normal individuals is largely controlled by a series of normal isoalleles or by closely linked modifiers. This variation is expected to affect the laboratory detection of the hemophilia A (HA) heterozygotes in addition to the X-inactivation-dependent mosaicism and the type of deficient mutant present in a given pedigree. The Sardinian pedigrees yielded 13 new cases of nonrecombinants between the loci for HA and glucose-6-phosphate dehydrogenase (G6PD), as well as four nonrecombinants between HA and Deutan color blindness. These findings bring to a total of 58 the number of scorable sibs and nonrecombinants thus far known for the linkage HA-G6PD. From such a figure it has been possible to infer that the 90% upper limit of meiotic recombination between the two loci is below 4%, thus justifying the application of the "linkage diagnostic test" for the detection of HA heterozygotes and the prenatal diagnosis of the hemophilic fetuses in families that segregate at both loci. In three out of the five HA pedigrees of our series that segregate also for G6PD or Deutan color blindness, the observed segregation of the combined phenotypes can be best explained by assuming the occurrence of a fresh mutation in the maternal grandfathers. Such a finding points out the opportunity to reevaluate Haldane's hypothesis of a possible higher incidence of X-linked mutations in the human male. It is anticipated that each of the issues addressed by the present study will be amenable to experimental verification as soon as suitable molecular probes become available to screen for common multiallelic DNA polymorphisms in the subtelomeric region of the X-chromosome long arm.