Advanced Placement (AP) Biology Practice Exam

What type of disorder is Huntington's disease?

1. Autosomal recessive disorder leading to physical deformities

2. X-linked disorder affecting males only

3. Standard dominant disorder resulting in neurodegeneration

4. Multifactorial disorder influenced by environment

The correct answer is: Standard dominant disorder resulting in neurodegeneration

Huntington's disease is classified as a standard dominant disorder because it is caused by a mutation in a single gene, specifically the HTT gene, which is located on chromosome 4. This gene encodes a protein called huntingtin, and the mutation leads to an abnormal expansion of CAG repeats. The presence of just one mutated copy of the gene is sufficient to cause the disease, which is characteristic of autosomal dominant inheritance. As a result, individuals with the mutated gene have a 50% chance of passing the condition to their offspring. The disease manifests primarily through neurodegenerative symptoms, which include movement disorders (chorea), cognitive decline, and emotional disturbances. These symptoms typically appear in mid-adulthood, although the age of onset can vary. This understanding highlights the mechanisms through which Huntington's disease affects the nervous system and distinguishes it from other types of genetic disorders that may involve different inheritance patterns or causes.