Question

for the pedigree shown here, the disorder is caused by a recessive (a) allele on the x chromosome. select the parents and/or grandparents that are clearly genetic carriers.

Explanation:

In X - linked recessive inheritance, females have two X - chromosomes (XX) and males have one X - chromosome (XY). A female carrier has one normal X - chromosome and one X - chromosome with the recessive allele. Since males only have one X - chromosome, affected males have the recessive allele on their single X - chromosome. The affected males must have received the recessive allele from their mothers. Looking at the pedigree, individual I - 2 is affected, so she must be a carrier as she has the recessive allele on one of her X - chromosomes. Also, individual II - 2 has an affected son (III - 2), so she must be a carrier as she passed on the recessive X - linked allele to her son.

Answer:

I - 2, II - 2