The fact that being homozygous recessive for the ccr5 receptor protects people from being infected with hiv, thus causing an increase in the recessive ccr5 allele, is an example of

Positive natural selection

Explanation:

Positive natural selection is fundamental to adaptive evolution and it is characterized by its ability to be passed on, that is heritable, and beneficial to a population.

your answer is "the 1950's"

c) 50% there are four chromosomes. after crossing over, 2 of the four have been changed. that means any gamete has a 2 in 4 chance or 50% chance of receiving the mutated chromosome.