Question

1. The CCR5-Δ32 allele, which evolved at least 700 years ago, appears to make human individuals resistant to HIV, which is a very recent disease. Write one hypothesis about why the CCR5-Δ32 allele might be common in European populations.

2. Explain how the concept of genetic drift (by bottlenecking) applies to the loss of genetic diversity of HIV when HIV is transmitted from an infected person to an uninfected person.

Answer 1

1. One hypothesis for the allele being common in European populations is that the allele might have arisen central Europe and increased to a higher frequency in the north because of a geographical gradient in selection intensity. There may be two causes for a gradient in selection- first, the selective advantage of delta 32 may have been larger in the north, and

second, it might be that a selective cost associated with the delta 32 allele may have been stronger in the south, and thus an overall selection intensity for delta 32 might have been weaker in the south and stronger in the north.

2. Genetic drift removes genetic variation from a population of living organisms. This might be due to chance disappearance of a gene as more individuals carrying that gene die or do not reproduce. This changes the frequency of a particular gene.

In the case of HIV loss of genetic diversity can be attributed to the fact that HIV carriers die due to the infection before they reproduce or before they are able to transmit the virus to healthy individuals. As a result of this, bottlenecking of HIV occurs, leading to the loss of genetic diversity.