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Hardy-Weinberg and Zebrafish

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A large population of zebrafish has been allowed to breed randomly for a number of generations in a laboratory.

After ten generations, 25% of the fish display a recessive genotype (dd), the same percentage as at the beginning of the breeding program. The rest of the fish show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants.

Which of the following is the MOST reasonable explanation of the fact that the frequency of the recessive genotype (dd) at generation ten is the same as the frequency of the recessive genotype (dd) at generation one?

A

Sexual selection occurred that favored allele d.

B

The genotype DD is lethal.

C

The population is undergoing genetic drift.

D

The two phenotypes are equally adaptive under laboratory conditions.