Final answer:
The expansion of GAA trinucleotide repeats on chromosome 9 is associated with Friedreich's ataxia, not Huntington's disease, which involves CAG repeats on chromosome 4. The correct option is d.
Step-by-step explanation:
The expansion of GAA trinucleotide repeats on chromosome 9 is associated with Friedreich's ataxia. This is a genetic condition that causes damage to the nervous system and movement problems. Huntington's disease, by contrast, is characterized by a different type of repeat expansion, CAG trinucleotide repeats, on chromosome 4.
Huntington's disease causes the degeneration of neurons in certain areas of the brain, leading to the physical, psychological, and cognitive symptoms that include involuntary spastic movement and difficulty controlling voluntary movement.
The defective protein in Huntington's disease contains long repeats of the amino acid glutamine, which is toxic to neural tissue. Myotonic dystrophy and Fragile X syndrome, on the other hand, are associated with repeat expansions in other genes. The correct option is d.