Final answer:
The penetrance for Huntington's disease (HD) in individuals with over 40 repeats of the Huntington allele is almost 100%, with symptoms usually manifesting in middle age. Those carrying the allele have a 50% chance of passing it on to their children.
Step-by-step explanation:
The penetrance of Huntington's disease (HD) in individuals with more than 40 repeats, such as those who are heterozygous for the dominant Huntington allele (Hh), is considered to be invariable, meaning that it is nearly 100%. These individuals will inevitably develop the disease, usually in middle age. Penetrance refers to the proportion of individuals carrying a particular variation of a gene (allele) that also express an associated trait or disorder. In the case of Huntington's disease, despite the late onset of symptoms, the genetic likelihood of passing the disorder to offspring remains significant. If one parent carries the Huntington allele, there is a 50 percent chance that each child will inherit the allele and thus the potential for developing the disease.