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Huntington’s disease is caused by a dominant genetic mutation in humans, meaning that inheriting even one copy of this mutant allele will cause a person to have the disease. Huntington’s is fatal — there is no cure. Though the mutation is present at conception, symptoms do not usually appear until a person is in their 30s or 40s. Earlier onset is possible but uncommon.

Why has natural selection failed to remove this harmful allele from the human gene pool?

A

The disease is not heritable, so its impacts on this generation have no effect on the next generation.

B

There is an ​insufficient variation of this trait within the human population for natural selection to act.

C

Natural selection has removed this trait in the past, but the mutation that causes it keeps reoccurring.

D

The disease does not significantly impact the fitness of individuals carrying this mutant allele.

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