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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?


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


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


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


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

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