Do you think presence of symptoms plays a role in this harmful gene persisting? why or why not?

Huntington's disease is an autosomal dominant disorder, which means that a person needs only one copy of the defective gene to develop the disorder. With the exception of genes on the sex chromosomes, a person inherits two copies of every gene — one copy from each parent.

Yes, subtle presence of symptoms contributes to persistence of the gene. People may not know they have the condition before having children, especially if an affected parent does not have clear signs or symptoms of the disease yet. It could also be a problem if the expecting parents do not know their birth parents or their medical histories.

Explanation:

substitution is the answer in this case