Answer:
The correct answer would be - d. With the end of genetic isolation gene flow took place
Explanation:
It is given that people of Troublesome Creek Kentucky were isolated and have a rare recessive allele that results in a blue tone to their skin. However, later these isolated people connected with the people from outside, and due which allow them to intermate. Intermating among populations from isolated and people from outside leads to gene flow.
Methemoglobinemia was present in the isolated population as they had both recessive alleles in their genotype and other hand people from outside might have heterozygous or both dominant allele genotypes that make them free from this disorder.
Due to the intermating and gene flow, the number of heterozygous conditions rises and after many generations slowly methemoglobinemia could have disappeared due to the masked recessive allele.
