Question

for the pedigree shown here, the disorder is caused by an autosomal recessive allele (a). which of the parents and/or grandparents can we be certain are genetic carriers? note: carriers does not include those that are affected.

Explanation:

Step1: Analyze offspring - parent relationship

Since the disorder is autosomal recessive (aa), affected offspring must receive one 'a' allele from each parent. In generation III, there are affected individuals (aa). Their parents in generation II (II - 1 and II - 2) must be carriers (Aa) as they are unaffected but have affected children.

Step2: Analyze generation I - generation II relationship

In generation I, individual 2 is affected (aa). When she has an unaffected child (II - 1), II - 1 must have received an 'a' allele from her. So, individual 1 in generation I must also be a carrier (Aa) to have an unaffected child with individual 2. Also, since II - 2 is a carrier and her parents in generation I are unaffected, at least one of them (I - 3 or I - 4) must be a carrier. But we can be certain that II - 1 and II - 2 are carriers, and I - 1 is a carrier.

Answer:

I - 1, II - 1, II - 2