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Hard to imagine P6 biology is asking questions about genetic inheritance.

Attached above is the answer to part a) with the diagrams filled up.

However, I believe there are errors in the question.

From a glance, the disease seem to be a X-linked recessive disease. However, the middle brother is a carrier. This is impossible if the disease is x-linked recessive, he can only be either suffer from disease X or not carry the disease X gene at all. It cannot be an autosomal recessive disease as well as Sally’s maternal uncle is not a carrier. The disease cannot be dominant in nature as there are carriers and hence they are heterozygous in genotype. Also, for Sally to suffer from disease X will thus require her father to be a carrier for the disease as explained that the disease cannot be dominant in nature due to the fact that there are carriers. Thus I believe the question has mistakes.

  1. Sally cannot be suffering from disease X if her father is not a carrier or homozygous for disease X.
  2. The middle brother cannot be a carrier as he can only be either a sufferer of the disease or not carrying the disease X gene.

b) Sally has 1 paternal uncle.

 

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