This is a pedigree of a family in which bleeder disease, an x-recessive disease, is present. I am asked which marker, A or B, the disease is linked to.
I've asked about this type of problem before: X-recessive disorders and genetic markers
The solution there was the fact that the families were unrelated so the disease being linked with a marker in one family did not correspond to the disease being linked to the same marker in another family.
The answer to this problem is "A". However this is ONE family and there are males in this family with the A marker on their only X-chromosomes who are unaffected and women with two A markers who are also unaffected. How would you explain that? They have the marker for the disease on both alleles but not the disease?