Genetics question

[MedGrl@2022]

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Sickle cell anemia is an autosomal recessive trait. The ability of homozygous recessives to survive and reproduce is greatly reduced. Which of the following must be true in order for the sickle cell gene to remain in the pool indefinitely?

A. Homozygous dominates must have increased fitness over heterozygotes.
B. Heterozygotes and homozygous dominates must have equal fitness.
C. Heterozygotes have increased frequency over homozygous dominants.
D. Heterozygotes must also have decreased fitness.

I choose B, my thought was that is the heterozygote had equal fitness to the homozygous dominates then the recessive allele that is responsible for sickle cell will remain in the population through carriers. EK says that: “C is correct. If the gene is actively selected against in one form, it must be actively selected for in another form or it will be eliminated from the population.”

Would answer B, result in a decrease of the allele responsible for sickle cell? Why is answer C more correct than answer B?

Thank you,
Verónica

 

[Leonardo Noto]

Sickle cell anemia is an autosomal recessive trait. The ability of homozygous recessives to survive and reproduce is greatly reduced. Which of the following must be true in order for the sickle cell gene to remain in the pool indefinitely?

A. Homozygous dominates must have increased fitness over heterozygotes.
B. Heterozygotes and homozygous dominates must have equal fitness.
C. Heterozygotes have increased frequency over homozygous dominants.
D. Heterozygotes must also have decreased fitness.

I choose B, my thought was that is the heterozygote had equal fitness to the homozygous dominates then the recessive allele that is responsible for sickle cell will remain in the population through carriers. EK says that: “C is correct. If the gene is actively selected against in one form, it must be actively selected for in another form or it will be eliminated from the population.”

Would answer B, result in a decrease of the allele responsible for sickle cell? Why is answer C more correct than answer B?

Thank you,
Verónica

The answer choices are worded very strangely, but here's what they're getting at. Homozygotes for the sickle cell mutation (two copies) get sick and die young in nature. Therefore, there must be a lot of reasonably healthy heterozygotes living their lives and reproducing to keep the sickle cell mutation in circulation. If heterozygotes were as sick as homozygotes ("have equal fitness;" Choice B) then everyone with this mutation would die young and the mutation would wither away through natural selection against it. Choice D is a little trickier--there's nothing in the question that tells you whether heterozygotes are healthy or if they are just healthier than homozygotes--all you can definitely deduce based on the question is that heterozygotes must be healthier than homozygotes and healthy enough to reproduce to keep the mutation floating around (and occasionally producing more homozygotes, who die young).

As an aside, people with sickle cell disease (homozygotes) have a really severe condition that is miserable and painful even with modern medical treatments. People with sickle cell trait (heterozygotes) are usually asymptomatic; however, because ~10% of African Americans have sickle cell trait, the homozygoes condition is by no means rare (although it can be avoided with genetic testing). In nature the sickle cell trait provides protection against malaria and in malaria endemic areas people with sickle cell trait are arguably healthier than those without (which is why it is still around)--but when two of them get together and produce a homozygote...therein lies the problem.

Dr. Leonardo Noto
www.leonardonoto.com

 

[MedGrl@2022]

If heterozygotes were as sick as homozygotes ("have equal fitness;" Choice B) then everyone with this mutation would die young and the mutation would wither away through natural selection against it.

I think for choice B, the homozygous dominates would have the healthy alleles since the question mentioned that "Sickle cell anemia is an autosomal recessive trait." Thus, the question really is: do heterozygotes need to be better off than the homozygotes dominant in order to maintain the recessive allele in the population? I know in real life that they are because they have protection against malaria, but if there was no protection, couldn't this recessive allele be continuously be carried by heterozygous carriers (since there would be no negative outcome for individual heterozygotes)?

 

[chopes31]

Answer choice C would be better worded as "Heterozygotes have increased fitness over homozygous dominants."

 

[advair250]

The way C is worded is correct. You want the Heterozygotes (Hh) to be more frequent than the homozygotes dominants (HH) because if that wasn't the case, the population overall would have less probability of having offspring that are carriers (Hh) and subsequently reproduce in future generations. (i.e. Hh x Hh = 50% chance of being Hh and 25% hh) Whereas if the frequency was less, there would be greater tendancy for HH x HH or HH x Hh ; as you can see less % of the gene pool are carriers, and significantly less affected.

The fitness of Hh vs HH as suggested in B, doesn't really allow you to infer proportions as C does. Assuming Hh and HH have equal fitness, all that is saying is that they are each equally as likely to survive and reproduce. It says nothing of the proportions of the genotypes in the population. Without having rough proportions of the gene pool, you can't say anything with regards to hh staying in the gene pool indefinitely.

Does that make sense? It is all about conserving the h allele.

 

[advair250]

I think for choice B, the homozygous dominates would have the healthy alleles since the question mentioned that "Sickle cell anemia is an autosomal recessive trait." Thus, the question really is: do heterozygotes need to be better off than the homozygotes dominant in order to maintain the recessive allele in the population? I know in real life that they are because they have protection against malaria, but if there was no protection, couldn't this recessive allele be continuously be carried by heterozygous carriers (since there would be no negative outcome for individual heterozygotes)?

The added protection against malaria, has no relevance to answering this question. The fundamental point of this question is to just see if you understand how frequency and proportions of genotypes are passed down to progeny, given the information that certain genotypes are detrimental.