Anopheles mosquitoes, which carry the malaria parasite, cannot live above elevations of 5,900 feet. In addition, oxygen availability decreases with higher altitude. Consider a hypothetical human population that is adapted to life on the slopes of Mt. Kilimanjaro in Tanzania, a country in equatorial Africa. Mt. Kilimanjaro's base is about 2,600 feet above sea level and its peak is 19,341 feet above sea level. If the incidence of the sickle-cell allele in the population is plotted against altitude (feet above sea level), which of the following distributions is most likely, assuming little migration of people up or down the mountain?

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Quizplus · Accepted Answer

The sickle-cell allele provides some resistance to malaria due to its effect on red blood cells, and it is more prevalent in populations that have historically experienced high rates of malaria. Since Anopheles mosquitoes cannot live above 5,900 feet, one would expect malaria to be less prevalent at higher altitudes. Therefore, the incidence of the sickle-cell allele would likely decrease with increasing altitude. Choice B shows a gradual decrease in frequency of the sickle-cell allele with increasing altitude, which is the most likely distribution. Choice A shows no change in frequency of the allele with altitude, which is unlikely given the relationship between malaria and altitude. Choices C and D show abrupt changes in allele frequency at certain altitudes, which are also unlikely.

Anopheles Mosquitoes, Which Carry the Malaria Parasite, Cannot Live Above