CAIE A Level Biology 9700 · Inheritance
In humans, the allele for normal haemoglobin (HbA) is dominant over the allele for sickle cell haemoglobin (HbS). A man who is heterozygous for the sickle cell trait and a woman who is also heterozygous plan to have children.
Construct a genetic diagram (Punnett square) for this cross. For each possible genotype, state the phenotype and explain how the mutation in the HBB gene leads to the abnormal haemoglobin protein.
6 marks, so the mark scheme is looking for about 6 separate creditworthy points. Making the same point twice in different words earns it once.
Describe means: Say what happens or what something is like, in order and with detail. You are not being asked why. If there is data, quote it: name the trend, then give figures with units at the points where it changes.
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