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?