Question

he distribution of the CCR5 mutation is found at high frequency in Northern Europe and at progressively lower frequency as we move south. This would be an example of which phenomenon studied in population genetics?

A) Gene flow  B) Drift C) Natural selection D) Mutation

Answer 1

Correct answer is A) Gene Flow

Explanation:

With regards to above question Options A ) Gene Flow and C) Natural selection can be correct. Natural selection was responsible for the origin of CCR5 mutation among North European population since it conferred advantage against several diseases such as Plague,Small pox, etc. Gene flow, on the other hand, was responsible for the spread of this mutation to the South Europe populations. Since the question is asking about the difference in the frequency of the mutation among the North and South Population , I am choosing Option A) Gene Flow. Please do consult your instructor about the meaning of the question.

Option A. is incorrect. The migration of Vikings from Northern Europe into the Southern Europe was a frequent event. CCR5 mutation conferred advantage to the individuals since they made these individuals resistant to several infections such as Black Death, Small pox,etc. The Vikings with Scandinavian descent who possessed these mutations carried them to the South populations. However the dispersal event wasn't very far due to which the allele frequency has a decreasing gradient as we move from North to South. The Southern places closer to North have comparatively higher allele frequency since they were easily invaded by the Vikings.

Option B. is incorrect . The age of the CCR5 mutation is around 700 years ago. Even though genetic drift can account for a quick increase in the allele frequency, it would not cause a neutral mutation to occur such high frequency in such short period of time. It is estimated that it would take genetic drift around 28 generations to push the allele frequency to such a higher level in Northern population. In addition, the allele should be rare in Southern population which is not the case.

Option C. is incorrect since natural selection was responsible for the origin of the mutation but genetic drift was responsible for the observed difference between North and South populations.

Option D. is incorrect since CCR5 mutation is itself a mutation event. Mutation cannot create the differences in allele frequency. They rather give birth to new unique alleles. Gene flow was responsible for the decreasing allele frequency to the South.