WeAreHD.org - The Social Support Network for People Living with Huntington's Disease

Heredity & HD

       As you all know Hd is a genetic probelm and it is inherited as an autosomal dominant disease. Autosomal dominat means all those with a faulty gene will be affected with it and if one of the parent is affected, the chance of transmiiting it to the offsprings is 50%.In the United States alone, about 30,000 people have HD. In addition, 35,000 people exhibit some symptoms and 75,000 people carry the abnormal gene that will cause them to develop the disease.

     A single abnormal gene produces HD. In 1993, scientists finally isolated the HD gene on chromosome 4. The gene codes for production of a protein called "huntingtin," whose function is still unknown. But the defective version of the gene has excessive repeats of a three-base sequence, "CAG." In the normal huntingtin gene, this sequence is repeated between 11 and 29 times. In the mutant gene, the repeat occurs over and over again, from 40 times to more than 80.

       This defect causes the resulting huntingtin protein to be malformed, prone to clumping in the brain and causing the death of nearby nerve cells. Cells of the basal ganglia, a brain area responsible for coordinating movement, and of the cortex, which controls thought, perception and memory, are most often affected.

       Since the gene that causes HD is dominant, each child of an HD parent has a 50-50 chance of inheriting the HD gene. The child needs only one copy of the gene from either parent to develop the disease. A person who inherits the HD gene, and survives long enough, will sooner or later develop the disease. If the child does not inherit the defective gene, the child will not get the disease nor pass the gene on to subsequent generations. Symptoms of HD generally appear in mid-life.