TBR bio 2 passage 1, question 5: simple genetics problem

[tdod]

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Why is the answer A and not B? Is an inidividual considered to have inherited a disorder even if he is just a carrier and doens't have the diseased phenotype?

 

[dnrs]

Those who are heterozygous for the sickle cell allele (AS) produce both normal and abnormal hemoglobin. Heterozygous individuals are usually healthy, but they may suffer some symptoms of sickle cell anemia under conditions of low blood oxygen, such as high elevation. Heterozygous (AS) individuals are said to be "carriers" of the sickle cell trait. Because both forms of hemoglobin are made in heterozygotes, the A and S alleles are codominant.

 

[tdod]

Those who are heterozygous for the sickle cell allele (AS) produce both normal and abnormal hemoglobin. Heterozygous individuals are usually healthy, but they may suffer some symptoms of sickle cell anemia under conditions of low blood oxygen, such as high elevation. Heterozygous (AS) individuals are said to be "carriers" of the sickle cell trait. Because both forms of hemoglobin are made in heterozygotes, the A and S alleles are codominant.

makes sense. But if that's the case, then why does the question refer to it as autosomal recessive?

 

[IslandStyle808]

makes sense. But if that's the case, then why does the question refer to it as autosomal recessive?

This question is really ambiguous. Here is a source that somewhat explains the whole thing. Use "edit" on browser>"find"> then type sickle cell (under title codominance of journal article).

http://www.ncbi.nlm.nih.gov/books/NBK21226/

In some senses it is recessive, in others it shows incomplete dominance. I really don't like doing TBR questions, because they can get ridiculous sometimes.

We don't even know how HbC acts. Is this a stand alone question or is this part of a passage?

 

[dnrs]

makes sense. But if that's the case, then why does the question refer to it as autosomal recessive?

They suffer from some symptoms but do not have sickle cell anemia.

 

[Czarcasm]

This question is really ambiguous. Here is a source that somewhat explains the whole thing. Use "edit" on browser>"find"> then type sickle cell (under title codominance of journal article).

http://www.ncbi.nlm.nih.gov/books/NBK21226/

In some senses it is recessive, in others it shows incomplete dominance. I really don't like doing TBR questions, because they can get ridiculous sometimes.

We don't even know how HbC acts. Is this a stand alone question or is this part of a passage?

It is unnecessarily ambiguous. It would help if the question clarified that the inherited mutations occur at the same loci instead of different loci. Totally changes the approach to this question. Knowing they are indeed different genetic mutations for the same loci (a case of multiple allelism), then you can approach the question as they did in their explanation. This is an important distinction, because it allows me to know if the mother carries two or one normal functional gene coding unit for the sickle cell trait. (Turns out, it's just one and the other allele is the HbC).

I don't think co-dominance or incomplete dominance is relevant here because either of those two scenarios represent a functional gene product that is expressed. But I suppose you could use that terminology here though since the mutation does show up phenotypically (although you wouldn't in a scenario where a mutation prevents any product formation at all). In this case, two mutants are being produced (no normal) and so the default is the genetic condition by default.