CAIE A Level Biology 9700 · Inheritance
In humans, pigmentation is controlled by the TYR gene. The TYR gene codes for the enzyme tyrosinase, which catalyses the production of melanin. Albinism is a condition characterised by a lack of melanin in the skin, hair, and eyes. A couple, both with normal pigmentation, have a child with albinism. Explain the genetic basis of this outcome, relating the TYR gene, its alleles, and the function of tyrosinase to the observed phenotypes.
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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