Part of a pedigree for the albinism trait is shown on the right. Only one of the individuals (III-3) is affected by albinism.Question 16(3 marks)SavedUsing the above pedigree, explain why Individual III-3 has albinism even though both of his parents lack this trait. In your answer, suggest the genotypes of the parents and Individual III-3.
Question
Part of a pedigree for the albinism trait is shown on the right. Only one of the individuals (III-3) is affected by albinism.Question 16(3 marks)SavedUsing the above pedigree, explain why Individual III-3 has albinism even though both of his parents lack this trait. In your answer, suggest the genotypes of the parents and Individual III-3.
Solution
Albinism is a recessive trait, which means that it can be carried in a person's genes without appearing in that person. For a person to have albinism, they must inherit two copies of the defective gene – one from each parent.
In the case of Individual III-3, even though both of his parents do not have albinism, they can still be carriers of the gene. This means they have one normal gene and one defective gene, but the normal gene is dominant, so they do not have the condition.
The genotypes of the parents (II-2 and II-3) would therefore be Nn, where N represents the normal gene and n represents the defective gene. They do not have albinism because the normal gene N is dominant over the defective gene n.
For Individual III-3 to have albinism, he must have inherited the defective gene from both parents, making his genotype nn. This is why he has albinism even though both of his parents lack this trait.
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