Q.Give an account of genetic control of Rh factor.

Step 1. State the antigen and its discovery: the Rh (D) antigen was found on red blood cells by Landsteiner and Wiener (1940) in rhesus monkey blood, and soon after in humans; carriers are Rh-positive, non-carriers Rh-negative, and (unlike ABO) anti-D antibodies are never present naturally, only appearing after actual exposure of an Rh-negative person to Rh-positive blood.

Step 2. Explain the Fisher-Race hypothesis: Rh type is controlled by three separate but tightly linked gene pairs — C/c, D/d, E/e — sitting at three adjacent loci on one chromosome (Fig. 4.1), so a genotype is written as a combination like CDE/cde; any genotype with at least one dominant D allele is Rh-positive, and only the double-recessive dd genotype is Rh-negative.

Step 3. Explain the competing Wiener hypothesis: instead of three loci, Wiener proposed a single Rh locus with eight possible alleles (R1, R2, R0, Rz, r, r1, r11, ry); any genotype carrying a dominant 'R'-type allele is Rh-positive, and the various double-recessive combinations (rr, rr1, rr11, rry) are Rh-negative. …

Present both the Fisher-Race and Wiener hypotheses for Rh inheritance and state their …

• Presenting only one of the two hypotheses when the text explicitly gives both as competing explanations. …