"H" Is for Heredity and Huntington's

By Margaret R. McLean

For background on this issue, see "G" is for Genes and Genomes

"I want to discuss the results of your Huntington's disease test," says the genetic counselor. Brad nervously gazes at the floor. If the result is positive, he will get Huntington's in the prime of his life and eventually die from it. If his test is negative, he has been spared. Genetic counselors help people like Brad understand the limits of genetic testing, possible emotional responses to test results, and the choices to be faced.

The human genetic instruction book is located on 23 pairs of thread-like chromosomes. The test Brad took looked for changes in the genetic code on chromosome 4. Such a variation in the language of genetics is called a mutation. Most mutations are harmless. However, sometimes a mutation can throw a monkey wrench into how the body works. This is what the mutation that causes Huntington's disease does.

Because chromosomes come in pairs, we have pairs of genes for nearly every genetic characteristic from hair color to susceptibility to cancer. But, if there are two genetic instructions--one for red hair and one for black hair--which is followed? It all depends on which gene is dominant and which is recessive. If one gene is dominant, then its instructions are followed.

The Huntington's disease gene is dominant. This means that only a single gene in the pair needs to carry the mutation in order for Brad to be affected. The change in this gene means that it gives erroneous instructions, which result in the breakdown in the section of the brain that controls movement, leading to a loss of muscle control and death.

We might think that Brad will be overjoyed if the counselor tells him that he has not inherited the Huntington's gene. While there is certainly relief in such news, Brad may experience a sense of guilt in being spared when his mother and uncle were not.

We might think that Brad would be tremendously upset to hear that he is living with the genetic time bomb of Huntington's. Perhaps, Brad may actually be relieved in knowing what lies ahead. Now, he can make decisions knowing that his future is limited.

"Brad, the test is negative." Brad sighs as he brushes away tears of relief. But, he wonders, "What about my brother? Has he escaped Huntington's, too?"

Margaret R. McLean is the director of biotechnology and health care ethics at the Markkula Center for Applied Ethics.

Return to The ABC's of Medical Ethics.

Posted in August 2006