Each child of a parent possessing a dominant gene has a 50 percent chance of inheriting the copy of that gene. In the case of Huntington s disease, a genetic test has been developed to diagnose presymptomatic individuals, although no treatment has yet been developed. Experience has shown, however, that in the absence of treatment, the actual percentage of people opting to take the test is much smaller. Most of the genetic diseases we know about today are recessive disorders. A recessive condition requires two copies of a gene to be expressed.
Cystic fibrosis is an example of a recessive disorder.
Thus, a person who develops cystic fibrosis has two abnormal copies of the gene, one inherited from each parent. While carriers will not develop the condition themselves, they have a 50 percent chance of passing on the gene to any one of their children. Children of parents who are both carriers have a 25 percent chance of inheriting both copies of the abnormal gene and developing the condition, and a 50 percent chance of being a carrier. By far the vast majority of diseases fall into a third category, the multigenic conditions, which are influenced by multiple genes working in conjunction with environmental factors. Technologies are being developed to test for the existence of thousands of genes simultaneously, in the hopes that someday we will be able to understand how these genes interact with each other.