In a small group of people living in a remote area, there is a high incidence of “blue skin”, a condition that results from a variation in the structure of hemoglobin. All of the “blue-skinned” residents can trace their ancestry to one couple, who were among the original settlers of this region. The unusually high frequency of “blue skin” in the area is an example of

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This question describes a small, isolated population where nearly all individuals with a rare trait (“blue skin”) trace their ancestry to a single couple. The key idea here is that in a small population, random fluctuations in allele frequencies can cause a trait to become common purely by chance when the population is descended from a small number of founders.

Option 1: mutation. While new mutations do introduce new alleles, it is unlikely that a new mutation reaching high frequency in all current residents would arise and spread so uniformly from one ancestral couple. Mutation alone generally starts at very low frequency and spreads slowly, not via a dramatic, founder-based pattern.

Option 2: genetic drift. This aligns with the observed pattern: a small founding group carries the allele for blue skin, and because the population originated from that couple, the allele frequency can become high by chance. This is a classic founder effect, a special case of genetic drift in a small population.

Option 3: natural selection. If blue skin conferred a clear survival or reproductive advantage, selection could increase its frequency. However, the description emphasizes ancestry and isolation rather than differential survival, making selection a less likely explanation for the high, founder-based frequency.

Option 4: sexual selection. A scenario where blue skin is preferred in mates could raise its frequency, but again this would require a consistent mating preference pattern across the population and does not specifically account for the single-couple origin.

Option 5: heterozygote advantage. This would maintain an allele at higher frequencies if heterozygotes had a strong reproductive edge, but the evidence provided points to inheritance from a founder pair rather than balancing selection maintaining the allele.

In summary, the most fitting explanation for the unusually high frequency of blue skin in this isolated group is a founder effect, which is a form of genetic drift arising from a small initial population derived from one couple. The other options describe mechanisms that could, in some contexts, influence allele frequencies but do not adequately match the described pattern of ancestry and isolation.

APBIO142-030 with labs S23 - CollegeBoard* AP Exam Prep: Multiple Choice

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