Jordan is a healthy twenty-six year old male. He is an only child, son of Jessica age 45 and Ryan 43. Jordan is married and has no children. Eight years ago, Jordan’s mother was diagnosed with Huntington’s disease, a hereditary disease that destroys neurons in the basal ganglia of the brain. This destruction of brain cells causes progressive deterioration of both mental and physical abilities.
The symptoms of the disease generally do not develop until a person is past 30. Early symptoms of the disease are often related to mood or emotional state, with afflicted individuals experiencing depression, irritability, apathy, or anger. Mental ability begins to decline early in the course of the disease and eventually leads to loss of cognitive and intellectual function. Uncontrolled movements of the hands, feet, face, and trunk appear early on but eventually decline and akinesia, or loss of movement, develops. The course of the disease, from observable onset to death, is about 15 years. Treatment is directed at the symptoms and there is no cure.
In 1993, scientists discovered the gene that causes Huntington’s disease. This discovery has led to the development of a very accurate presymptomatic test for Huntington’s disease. The presymptomatic test determines if an individual has the gene for Huntington’s disease before the person develops any of the symptoms of the disease. Jordan contacted the Huntington Disease Society of America, obtained the location of a testing center, and is currently taking part in a testing program. As part of the testing program, he underwent a complete neurological examination and no symptoms of Huntington’s disease were apparent. Jordan is currently in pre-test counseling. During this component of the testing program he will learn about the disease, his risk, and the testing procedure. Jordan can then decide if taking the test is in his best interest.
Huntington’s disease is caused by a single abnormal, or mutant, gene that is located on chromosome 4. The mutant form of the gene has a sequence of three DNA base pairs repeated dozens of times. This three base pair sequence, CAG, also occurs in the normal form of this gene, but is not repeated as often. DNA from a blood sample is used to perform the presymptomatic test, which examines the base sequence of the gene for the disease and counts the number of CAG sequences. Individuals with more than 40 of these sequences in their gene will develop Huntington’s disease.
Huntington’s disease is an example of autosomal dominant inheritance. This means that the gene in question is located on an autosome, and the form of the gene that causes the disease is dominant over the normal form.
1. What are the chances that Jordan develops Huntington’s disease later in his life if the pre-symptomatic test identifies him as heterozygous for this particular gene? 2. What are the chances that Jordan’s children will inherit the mutant form of the gene from their father if Jordan is identified as heterozygous? 3. Would you expect males and females to be affected by this disease in equal numbers? Why? 4. Why are dominant disorders seldom lethal? What is special about Huntington’s disease that maintains this dominant and lethal disorder in the population? 5. Would you take the pre-symptomatic test? Why or why not?