Question

Huntington is a dominant disorder. A man is heterozygous for the Huntington gene (Hh). He marries a woman who is (Hh). Will the man or woman develop the disease? What is the probability that their first child will develop the disease? Draw a Punnet Square.

Answer 1

Huntington is an inherited disorder that results in the death of brain cells. The disease is inherited in an autosomal dominant pattern which means that one copy of the altered gene in each cell is suficient to precipitate the disorder. The person who is affected, usually acquired the altered gene from their affected parents. As the huntington gene(Hh) is passed from one generation from another the CAG trinucleotide repeat increases in size. The larger size of the repeats lead to early developments of signs and symptoms. Example:people with CAG repeats of 40-50 develop diseases in adult stage, whereas in juvinile cases the CAG repeats are more than 60. Usually the CAG repeats of 27-35 repeats of CAG donot develop the disease but their children will have the risk of development of the disorder. Therefore in the above case, where both the man and women are carrier their chances of developing the disease will depend on their CAG repeats. They may or may not develop the disease.

The children of the parents with affected gene or carriers of the altered gene will have 50 % chances of developing the disease. But in the above scenario where both the parents have altered gene present i.e both are carriers, therefore there is 75% chances for children to develop the Huntington disease.

In the above punnett square, H is the dominant trait. There is HH and 2 Hh which makes the ratio as 1:3. Therefore the chances is 75%.