Rh null phenotypes are not due to a gross deletion and can occur on different Rh genetic backgrounds.

Alu element-primed PCR was performed on genomic clones containing human RH blood group genes. When used as a probe, the Alu PCR product detected a restriction fragment-length polymorphism which is in complete linkage disequilibrium with the Rh C/c serological polymorphism, irrespective of the Rh D or E serological type it is coupled with. This provides the opportunity to type individuals for their RH C gene directly at the DNA level. RFLP analysis of two individuals with the amorph Rh null phenotype revealed that in one case this phenotype occurred on an RH C background, whereas in the other it was on an RH c background. Taken together these results indicate that the Rh C/c polymorphism has arisen only once, but that the amorph Rh null phenotype, although exceedingly rare, is the result of at least two independent mutations.