ABSTRACT
Rhesus factors, so important in producing successful blood transfusions, are inherited in a very straightforward manner but, unlike the previous example of MN blood groups, rhesus exhibits a system of completely dominant and recessive expression. Rh positive phenotypes are dominant over Rh negative. The gene alleles of the rhesus system come in three different types so closely linked together that they are inherited as a block. The presence of even a single dominant allele in any of these three genes results in an Rh positive phenotype. Because of this weighting against the inheritance of a complete set of recessive alleles, the Rh negative phenotype is relatively rare. There is also something of particular interest in the level of this rarity in that it varies from ethnic group to ethnic group. So, for example, a large study in Sweden showed that approximately 85% of those tested were Rh positive and 15% were Rh negative. Smaller studies in the US came out with a similar figure for white Americans, but black Americans were about 90% Rh positive and 10% Rh negative. An even more extreme variation was found among native American Indians from Oklahoma, where 99% were Rh positive and only 1% Rh negative. The comparison between white Americans and Swedes is as would be expected, since white Americans would have mostly originated from Europe.
