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This question is from "Concepts of Genetics," Klug & Cummings, 10e.

"Thalassemia is an inherited anemic disorder in humans. Affected individuals exhibit either a minor anemia or a major anemia. Assuming that only a single gene pair and two alleles are involved in the inheritance of these conditions, is thalassemia a dominant or recessive disorder?"

I know from a quick Google search that the disorder is recessive, but how would I come to this conclusion based on the question alone?

Edit: I've spoken to my professor, and he shared his opinion that is essentially the same as canadiener's answer. Thanks to everyone that replied!

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  • $\begingroup$ I would say neither, as ">either a minor anemia or a major anemia" implies two phenotypes. This is co-dominance if I am not mistaken. Dominant would mean always major anemic (either homo or heterozygous) when affected, recessive would mean only major anemic (homozygotic). Perhaps the geneticists among us can look into this? $\endgroup$ – AliceD Jan 21 '15 at 1:55
  • $\begingroup$ I was under the same impression about codominance, but the text splits up inheritance patterns by the usual dominant/recessive in one chapter and all the modified patterns in another. This question is listed in the first of these two, so I assume I'm given enough info in this chapter to be able to answer, but I just don't see the reasoning. $\endgroup$ – user170231 Jan 21 '15 at 1:58
  • $\begingroup$ no answers in the back o/t book? ;) $\endgroup$ – AliceD Jan 21 '15 at 2:24
  • $\begingroup$ Not for this one, unfortunately. Only evens get answers >_<. I'll consult the professor when I can. $\endgroup$ – user170231 Jan 21 '15 at 2:33
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    $\begingroup$ I think in general, we prefer to use "incomplete dominance" when the heterogeneous phenotype is in between the two homozygous conditions, and "co-dominant" when the het show both distinct phenotypes. If a red and white flower cross yields pink flowers, that would be incomplete dominance. A and B blood types having an AB child is co-dominance. But these terms don't really tell you much about the underlying molecular genetics $\endgroup$ – swbarnes2 Jan 21 '15 at 20:32
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Given that this is a text book question about classical genetics, you can safely disregard any reality about the disorder.

I'd argue that the two alleles show incomplete dominance. The mild anemia, which is intermediate between healthy individuals and those with severe anemia, can be attributed to heterozygous individuals. In this case, the expression of the functional allele is not enough to compensate for the nonfunctional allele, producing the mutant phenotype.

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