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MCAT Biology Question — Genetic Drift

Which of the following is (are) true regarding genetic drift?

I. Genetic drift has more pronounced effect on alleles with a low allele frequency than on alleles with a high allele frequency, regardless of population size.
II. Genetic drift is due to random sampling during reproduction.
III. The more isolated a population is, the less pronounced the effect of genetic drift is.


A. I only

B. II only

C. I and II only

D. II and III only

Click for Explanation

When a population reproduces, the alleles in the offspring are a random sample of the alleles in the parental generation. Due to this, changes may occur in the genetic composition of a population due to chance. These changes are genetic drift. Thus (II) is correct, eliminating choice (A).

Genetic drift is less pronounced in large populations that are connected with other populations. When a population is isolated, there is a higher likelihood that a particular allele can disappear entirely, or become entirely fixed. Thus, (III) is false, eliminating choice (D).

Finally, the effect of genetic drift is larger when an allele is less common. The fewer copies of an allele there are, the more likely it is that the allele may disappear entirely due to genetic drift. While genetic drift has a bigger impact on smaller populations, within any given population (regardless of size), the effects of genetic drift will be strongest on alleles with a low allele frequency. Thus (I) is correct and choice (C) is the right answer.

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