CAIE A Level Biology 9700 · Inheritance
A couple, both heterozygous for the sickle cell allele (HbA HbS), are planning to have children. The allele for normal haemoglobin (HbA) and the allele for sickle haemoglobin (HbS) show codominance.
Explain the expected genotypic and phenotypic ratios of their offspring, and describe how the genotypes give rise to the phenotypes at the molecular level.
5 marks, so the mark scheme is looking for about 5 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
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