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HIV Resistance

APBIO-ZEMJO5

The CCR5 gene codes for a protein on the surface of T cells. The CCR5 protein is used by Human Immunodeficiency Virus (HIV) to enter T cells, which ultimately leads to Acquired Immunodeficiency Syndrome (AIDS). AIDS was first reported in the European population in the early 1980s.

About 3,000 years ago in Central-Western Europe, a mutation occurred that prevents normal expression of the surface protein. People with one copy of the mutated allele have delayed onset of AIDS. Homozygotes for the mutation are resistant to HIV infection.

Evidence suggests that the trait also protects individuals from the smallpox virus. Smallpox was common in the European population from the Middle Ages until the early 1900s, causing several major epidemics with high death rates. It was eradicated through widespread vaccination.

The frequency of the allele is 16% in Scandinavia and 4% in Greece.

What are the expected frequencies of HIV-resistant​ individuals in each population and which evolutionary explanation for the difference is MOST plausible?

A

The frequency of resistant individuals is expected to be 0.0256 in Scandinavia and 0.0016 in Greece. Environmental factors caused historically higher mutation rates in the Scandinavian population, leading to the formation of more copies of the resistance allele.

B

The frequency of resistant individuals is expected to be 0.0256 in Scandinavia and 0.0016 in Greece. Higher rates of smallpox infections in Scandinavia created a selection pressure for the resistance allele.

C

The frequency of resistant individuals is expected to be 0.2688 in Scandinavia and 0.0768 in Greece. HIV is causing natural selection for the resistance allele.

D

The frequency of resistant individuals is expected to be 0.2688 in Scandinavia and 0.0768 in Greece. Scandinavian populations were isolated for much of European history, leading to genetic drift and increasing the frequency of the resistance allele.