Huntington’s disease (HD) is prevalent South Africa. Most of the families with HD have Dutch or British ancestry, which suggests they inherited the European HD allele. HD is believed to have first arrived in South Africa more than 300 years ago when Dutch colonists settled into the Western Cape in the 1600s. Thus, the prevalence of HD in South Africa is the result of the founder effect. Mutations in the HTT gene cause Huntington’s disease. The HTT gene provides instructions for making a protein called huntintin. This protein plays a role in nerve cells in the brain. The HTT mutation that causes HD involves a DNA segment known as a CAG trinucleotide repeat. This segment is made up of a series of three DNA building blocks that appear multiple times in a row. Normally, the CAG segment is repeated 10 to 35 times within the gene. In people with HD, the CAG segment is repeated 36 to more than 120 times. An increase in the size of the CAG segment leads to the production of an abnormally long version of the huntintin protein. This causes the dysfunction and eventual death of neurons in certain areas of the brain. The more repeats, the more serious the disease is.

HD is inherited in an autosomal dominant pattern. There is both an adult onset (after reproductive age) and a juvenile onset pattern. People with adult onset typically have 40 to 50 repeats, while people with the juvenile form tend to have more than 60 repeats. Individuals who have 27 to 35 repeats do not develop HD, but they are at risk of having children who will develop the disorder. As the gene is passed on to offspring, the size of the repeat may lengthen into the range associated with HD.

Predict what will happen to the allele frequencies for Huntington’s disease in South Africa if there were a new mutation to the HTT gene that causes a reduction to the CAG trinucleotide repeat. Justify your prediction with evidence. Your response should include links to the sources of information gathered.