Huntington's disease is caused by a dominant genetic variant, meaning that a person only needs 1 copy of the disease-causing variant in order to have the disease. Symptoms usually appear when people are in their 30s or 40s, so some people have kids before they realize that they are affected and could pass the genetic variant to their children.A couple decided to have 3 children. Unbeknownst to them, one of the parents carries the genetic variant for Huntington's disease, so each child has an independent probability of 1/2 of developing Huntington's disease.

a. What is the probability that exactly 2 of their children will develop Huntington's disease