Final answer:
The expanded repeat in the Huntingtin gene is a CAG trinucleotide sequence found in the first exon on the short arm of chromosome 4, and it usually exceeds 36 repeats in individuals with Huntington's disease.
Step-by-step explanation:
The repeat that is expanded in the Huntingtin gene is a trinucleotide sequence consisting of the nucleotides cytosine, adenine, and guanine (CAG). This sequence is located at the very beginning of the gene, in a region known as the first exon. The Huntingtin gene itself resides on the short arm of chromosome 4. Normally, the CAG repeat ranges from 10 to 35 repeats in the population, but in individuals with Huntington's disease, the number of CAG repeats is significantly increased, generally exceeding 36 repeats.