In Huntington's disease, earlier age of onset is more often associated with inheritance from which parent?

• A. Father
• B. Mother
• C. Sibling
• D. Grandparent

Answer: (A)

Huntington’s disease shows anticipation: the number of CAG repeats in the HTT gene correlates with age at onset—the more repeats, the earlier the disease tends to appear. These repeats are particularly unstable during sperm formation, so when the mutated allele is transmitted by the father, the repeats often expand more in the offspring. That greater expansion in paternal transmission leads to an earlier onset in the child compared with the parent, which is why earlier onset is most often seen with inheritance from the father. Transmission from the mother can still cause disease, but the repeat expansion tends to be less pronounced, so onset is not as typically earlier. Siblings and grandparents are not the direct basis for this pattern; the critical factor is which parent passes the mutated allele.