Sample Exam B/B #38

[kryptonxenon]

Question 38: A mother and father with normal color vision have a son who is color-blind with green pigment absent. What genotype did the mother most likely have?

Why are there two genotypes listed for the mother? How do you interpret the answer choices?

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[theonlytycrane]

Those are the alleles on each of her chromosomes.

Green color deficiency is X-linked. So the gene in their son will have come from the mom. The specific phenotype expressed by the son is "green pigment absent". Look for this phenotype in the table and match the genotype to be G- / R+. This eliminates choices B and D because those choices don't have a G- allele. C shows the allele on one of the mom's chromosomes. A could work if the alleles crossed over, but the gene coming from C would be more likely.

 

[NextStepTutor_2]

Question 38: A mother and father with normal color vision have a son who is color-blind with green pigment absent. What genotype did the mother most likely have?

Why are there two genotypes listed for the mother? How do you interpret the answer choices?

Hi @kryptonxenon Table 1 and the paragraph above contain all you need to answer this question. The "tricky" part to this Q is that the AAMC asks for the MOST LIKELY genotype. this is something the AAMC does very often so that they can throw out more than 1 scientifically valid answer, while still only having 1 correct answer to the question.

You are told in paragraph 2 that Green pigment loss is X-linked. Normal vision (+) is dominant to lack of pigment (-). Both parents have normal vision but we can start with dad first. Dad cannot pass any defective X alleles onto his son, so he must be G+R+. Now you can eliminate choices B and D.

Onto mom. Mom we are told has normal vision but passed on lack of green pigment (G-) onto her son. Both A and C have that allele present but choice A is not as good an answer (though it might still be possible) considering the only information we have on her is that HER vision is normal and she passed on a lack of green pigment to her son. As far as the information we have, there is no reason to suspect she has an allele for lacking red pigment (R-).

Long story short, when you see works like MOST or BEST on the MCAT, be wary of tempting answers that may be logical or scientifically sound, but do not answer the question you were asked.

Hope this helps, good luck!