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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?

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  • $\begingroup$ Look at my answer in this post biology.stackexchange.com/questions/30885/…. That should answer why some males can carry the A marker and not get the disease. $\endgroup$
    – cagliari2005
    Commented Jun 8, 2015 at 4:04
  • $\begingroup$ So I:1 is a recombinant and the sick allele is the A allele from the mom? $\endgroup$
    – Paze
    Commented Jun 8, 2015 at 9:15
  • $\begingroup$ Your account should be closed for asking several homework questions every single day. $\endgroup$
    – tommy.carstensen
    Commented Jun 11, 2015 at 7:47

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Remember: A and B are genetic markers that might be linked to the disease gene. Neither A nor B are actually the hemophilia disease mutation (or gene), A or B are proxies, or substitutes for the mutation in the disease gene. Since it is an X-linked recessive, it is only the Mother's two X chromosomes that are candidates. You can completely ignore the Father's copy of X that also happens to carry the A marker. In the F1 generation every male who received the copy of the Mother's X marked with A died, and every male who received her X marked with B survived. Therefore her X marked with A is linked to the mutation.

Those other AA females are either heterozygotes, or outsiders who married into the family. The outsiders do not carry the mutation.

Perhaps your confusion is due in part to the fact that some key facts are not explicit in the pedigree diagram from your instructor's course materials? Try drawing the various genotypes out like this:

Parental Genotypes

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