Genetics recessive, x-linked allele problem, need help!?

Should be B

A - the mother is a carrier and the father is color blind.

It looks like this girl inherited the X chromosome with the allele from her mother and the other from her colour-blind father (as he has no other X chromosome).


Hope that Helps

A) The mother is a carrier, the father is colorblind.

--For Emma (a girl) to have colorblindness, she would have to inherit two faulty versions of the X chromosome, meaning one has to come from mom and one has to come from dad.
--We know the father has a faulty X chromosome. Males are hemizygous for X, meaning they only have one copy, and therefore whatever the have will show up despite being recessive. The father MUST be colorblind.
--We know the mother has to have at least one faulty X, so to know the rest you look at the brother. He is NOT colorblind, meaning the X he inherited is fine. He inherits his Y from dad, so the X MUST come from mom.
--THEREFORE, the mother has one good X and one busted one, meaning she is a carrier and not colorblind while the father is colorblind.

The father determines whether or not the offspring will be colorblind, as he contributes either the 'X' or the 'Y'.

To be colorblind, a girl needs the gene for colorblindness from BOTH PARENTS, meaning BOTH PARENTS need to give her an 'Xc'.

Emma gets one 'Xc' from her mother, and one 'Xc' from her father--this means her father HAS TO HAVE THE GENE FOR COLORBLINDNESS ON HIS X, meaning he HAS TO BE COLORBLIND. This eliminates choices B and D (he is not just a carrier, he is colorblind).

However, since her brother is not, this means the 'X' he received from his mother does NOT have the gene for colorblindness, meaning his mother is JUST A CARRIER (she did have one Xc, which she passed on to Emma, but Emma's brother got the X which was not affected).

This means choice A is correct--their mother is a carrier, and their father is colorblind.

Hope this helped you understand!