d. Zygosity of Rh-Positives.
(1) Assumptions about Rh genotype are based on knowledge of how
frequently individual genes occur in the population (see Table 2-10). The Rh-negative
trait is most often a result of the gene that also specifies hr'(c) and hr"(e); much less
often rh'(C) or rh"(E) may accompany this trait. A person with positive cells that have
only Rho(D), rh'(C), and hr"(e) is most likely to have the genotype R1R1 (CDe/CDe). He
could be heterozygous, with the genotype R1r' (CDe/Cde), but this is less likely,
because r' is infrequent in most populations. A White person whose cells have Rho(D),
hr'(c), and hr"(e) would probably be heterozygous (Ror) because r is so much more
common than Ro in Whites; but a Black with that phenotype would probably be
(2) A person with cells which react with all five major antiserums (see line 3
in Table 2-11) could have any one of a variety of genotypes. He would probably be
homozygous Rh-positive, meaning that his two genes, although different from each
other, both code for Rh-positive. The most probable genotype in any population group
is R1R2 (CDe/cDE). Genotypes heterozygous for Rho(D) require that 1 or 2 uncommon
alleles be present, for example R1r" (CDe/cdE), R2r' (cDE/Cde), or Rzr (CDE/cde).
Table 2-11 gives the frequency of the more common Rh-positive genotypes in the White
population. In other racial groups, there is less likelihood that Rh-positive phenotypes
result from heterozygous genotypes, because Rh-negative genes are less frequent.
2-20. Rh VARIANT (Du)
(1) Not every Rh-positive cell sample reacts equally with every anti-Rho(D)
serum. Most specimens show clear-cut agglutination after immediate spin, and can
easily be categorized as Rh-positive. Cells which are not immediately agglutinated
cannot so readily be categorized as Rh-negative, because some Rho(D)-containing cells
react to a variable degree with the antiserum, but are not agglutinated. Since the
antigen is present, the cells are indeed Rh-positive, and additional testing is necessary
to demonstrate it. These types of reactivity are called Du.
(2) The Du phenotype can arise from several different genetic
circumstances. Some genes for Rho(D) seem to code for a weakly reactive antigen,
and this trait can be shown to follow regular patterns of inheritance. In Blacks, the trait
is fairly common, often appearing as part of the gene Ro (cDe). Du is considerably less
common in Whites. When it occurs, it more often is found as a variant of the R1 (CDe)
or R2 (cDE) genes, which are more common in the White population than Ro (cDe).
Some of these genetically determined Du may react weakly with most anti-Rho(D)