Kaplan FL#1 Bio #41 question

[KungFuPanda123]

The question: When two different dominant alleles at the same locus contribute to a genotype, the phenotype that arises is best described by which of the following?

A. Incomplete dominance
B. Codominance
C. Penetrance
D. Expressivity

Answer: B
Explanation from Kaplan: The perfect example of codominance is blood type antigens - when two alleles such as the A antigen and the B antigen exist, both are expressed and the erythrocytes are classified as "AB."
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In response to the Kaplan answer: When 2 different dominant alleles are at the same locus, how can you predict that it'll be shown phenotypically as codominance versus incomplete dominance? Aren't incomplete dominance and codominance just 2 ways of showing what happens in cases when 1 allele is not 100% dominant over the other? Seems like there's no way to know for sure that the answer is "Codominance" as opposed to "Incomplete Dominance."

General thoughts/reasoning: Is there any way for a situation where 2 "dominant alleles" are not mutually dominant (i.e. as is what is the case for codominance/incomplete dominance)? In other words, what I mean is is, is it possible for one "dominant allele" to be less dominant than another? (Maybe this could be the case if one of the dominant alleles is considered dominant to a 3rd allele but is not dominant to the allele mentioned in the question) If this were the case, could Penetrance be the correct answer, since penetrance would determine which allele is responsible for the phenotype that arises?

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Overall, I didn't like the wording of the question, but I hope I can get some clarification on Kaplan's answer and if there are any issues with it. Thanks!

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[azor ahai]

It's codominance. Pretty straightforward definition.

 

[sazerac]

For the benefit of the viewing audience, how would the definition of incomplete dominance be different?

 

[Cawolf]

When I read this I was confused as well.

I would've picked codominance though - due to that being the "standard" both contributing to phenotype.

I think that if I read "one contributes more than the other" I would lean towards incomplete dominance. Question is too vague IMO though - standard of what I have seen of Kaplan. Not impressed and not aligned with the question style I saw on the MCAT.

 

[krukshanks]

"if a heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is said to exhibit no dominance at all"
http://en.wikipedia.org/wiki/Dominance_(genetics)#Incomplete_dominance
According to Wikipedia, if the phenotype of incomplete dominance is right in the middle of the two extreme phenotypes, then neither allele should be called dominant. If you take this into account then I guess codominance would be a better choice, but I agree this is a badly written question.

 

[NextStepTutor_1]

I agree as well. The question stem could have been a bit clearer since technically in co-dominance and incomplete dominance both dominant alleles play some part. As @Cawolf mentioned, I think I too would have chosen co-dominance since there is not direct clue to think the two alleles are "mixing", if you will, and forming a phenotype that is different from that caused by each allele separately. I think of co-dominance as a yin-yang and incomplete dominance is terms of mixing paint to form a new color.
Mcat-review here can help if this is confusing, though I again agree that the question is ambiguous.
http://mcat-review.org/genetics.php