Describe how natural selection can sometimes lead to the persistence of harmful or even lethal alleles. include an example.

Heterozygosity of genes sometimes allows natural selection to maintain deleterious alleles. Sickle cell anemia is only exhibited by people who carry both alleles as recessive.
The recessive/recessive allele can become advantageous in a different environment or time. Sickle cell recessive allele protects their carrier against acquiring malaria.

g. 2/3

Explanation:

Huntington disease is a autosomal dominant disorder. This disorder can be inherited and there is 50% chance that a child may get infected from his father's disorder. Jeff is 50 and has not shown any disorder symptoms. There are 2/3 people who do not show any symptoms before age of 50. Therefore there is 2/3 chance that Jeff will eventually show symptoms of Huntington disorder.