Huntington disease is a trinucleotide repeat disease in which the CAG repeats are typically in which range?

• A. Autosomal dominant disease
• B. Autosomal recessive disease
• C. X-linked disorders
• D. Trinucleotide repeat disease where CAG repeats are 40-180 times; normal is 10-35 times

Answer: (D)

Huntington disease arises from an expansion of CAG repeats in the huntingtin gene, and the number of repeats distinguishes normal from disease-causing alleles. Normal individuals typically have about 10–35 repeats, while disease-causing alleles are expanded well beyond that, commonly in the range of 40–180 repeats. The larger the expansion, the more likely and earlier the disease tends to present due to the toxic effect of the polyglutamine tract. So the best answer reflects that disease-associated repeats are around 40–180, with normal alleles around 10–35. (Huntington’s is autosomal dominant, but the question is about the repeat length.)