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AP Biology Question Review 2 - Sickle Cell Traits

Our second installment into a question review for AP Biology finds us covering the concept of trait inheritance for the sickle cell disease. Our first AP Biology review dealt with the history of angiosperms and what time period did the plant species first emerge in. With a completely new concept to delve into, lets get into it with the question prompt below:

Sickle cell anemia is caused by mutant hemoglobin DNA. The sickle cell allele creates an altered mRNA codon that produces hemoglobin containing valine rather than glutamic acid. If a person inherits both alleles for the sickle cell trait, his hemoglobin will polymerize under low oxygen conditions (i.e., elevated physical activity). This can result in brain damage, paralysis, kidney failure, and other very serious physiological problems.

Heterozygotes for the sickle cell trait have increased resistance to malaria. If malaria were eradicated and effective treatment for sickle cell anemia made universally available, what would be the expected effect on the sickle cell trait?

Your answer choices are:

  • The frequency of the trait would decrease.
  • The frequency of the trait would remain constant.
  • The frequency of the trait would increase.
  • The frequency of homozygous individuals would decrease, and the frequency of heterozygous individuals would remain constant.

The answer to this question is as follows. If malaria were eliminated, selection for the sickle cell allele would stop. Individuals with the allele no longer would have a greater chance of survival than those without it. At the same time in this scenario, homozygous individuals would receive treatment that would ensure their survival through reproductive age and beyond. As it would no longer be selected for or against, the sickle cell allele's frequency would remain constant. In essence, eliminating the disease can be done, however, the sickle cell trait will always remain.

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Image Source: http://www.cdc.gov/ncbddd/sicklecell/traits.html

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