Huntington's disease inheritance risk and genetic testing implications

Runs Family

It Runs in the Family

What is the chance that Melissa will develop HD and what could she expect if she did develop the disease?

Huntington's Disease is caused by a dominant mutation in a gene, meaning that if one or both of the gene that carries this mutation has the Huntington's mutation, the disease will manifest physically (NIH 2011). In other words, if one of a person's two biological parents had or has Huntington's Disease, there is at minimum a fifty percent chance of an offspring inheriting the disease, and inheritance will lead to expression if given enough time (May Clinic 2011). Because Melissa does not have access to any pertinent information other than that her mother had the disease, calculating her potential for developing the disease becomes much more complex, as all alternatives must be considered. The Punnet squares below help to illustrate this:

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These six Punnet squares show all potential parents and offspring in terms of Huntington's disease, and any but the last (the bottom right) could represent the union of Melissa's parents (her mother was not homozygous recessive, as she developed the disease, but that's all that is known). Of the twenty potential offspring in the five pairings that could represent Melissa's parents, only three do not carry the Huntington's mutation, giving Melissa a 17/20 or 85% chance of developing Huntington's. If she does have Huntington's, there is a 50% chance that she is homozygous dominant (will definitely pass on the trait to her offspring) and 50% heterozygous (50% chance of passing it on), for a 75% total chance that Melissa's offspring will have Huntington's if Melissa has it and a 63.75% (85% x 75%) total chance that her offspring will have it.