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The RhD antigen is a protein found on red blood cells in humans - VCE - SSCE Biology - Question 5 - 2014 - Paper 1
Question 5
The RhD antigen is a protein found on red blood cells in humans. The presence of the protein is genetically controlled. Possible phenotypes and genotypes with respec... show full transcript
Worked Solution & Example Answer:The RhD antigen is a protein found on red blood cells in humans - VCE - SSCE Biology - Question 5 - 2014 - Paper 1
Step 1
With reference to the father’s possible phenotypes(s) and genotype(s), explain how a Rhesus-negative mother could be pregnant with a Rhesus-positive fetus.
Answer
For the mother to be Rhesus-negative (genotype dd), the father must carry at least one D allele for the child to be Rhesus-positive. The possible genotypes for the father would be DD (homozygous Rhesus-positive) or Dd (heterozygous Rhesus-positive). If the father is Dd, there is a 50% likelihood that he can pass on a D allele, resulting in a Rhesus-positive fetus (genotype Dd) even though the mother is Rhesus-negative.
Step 2
Explain, by referring to the immune response, why the fetus in a second pregnancy is at far greater risk of HDN than the first Rhesus-positive fetus.
Answer
In the first pregnancy, if the Rhesus-positive fetus has D antigens, the mother's immune system may produce antibodies against those antigens only if the fetal blood enters her circulation. This initial exposure sensitizes the mother. In a subsequent pregnancy with another Rhesus-positive fetus, the maternal antibodies can cross the placenta and attack the fetal red blood cells, leading to Hemolytic Disease of the Newborn (HDN). This increased risk is primarily due to the memory response of the immune system, where IgG antibodies produced during the first exposure can respond more aggressively in future pregnancies.