Genetic Diseases - Tay-sachs Disease

TSD is a fatal genetic disorder in children that causes the progressive destruction of the central nervous system. It is caused by the absence of an important enzyme called hexosaminidase A (hex-A). Without hex-A, a fatty substance builds up abnormally in the cells, particularly the brain's nerve cells. Eventually, these cells degenerate and die. This destructive process begins early in the development of a fetus, but the disease usually is not diagnosed until the baby is several months old. By the time a child with TSD is four or five years old, the nervous system is so badly damaged that the child dies.

Symptoms of Tay-Sachs Disease

A baby with Tay-Sachs disease (TSD) seems normal at birth and usually develops normally for about the first six months of life, but then development slows. The child begins to regress and loses skills one by one—the ability to crawl, to sit, to reach out, and to turn over. The victim gradually becomes blind, deaf, and unable to swallow. The muscles begin to atrophy, and paralysis sets in. Mental retardation occurs, and the child is unable to relate to the outside world. Death usually occurs between ages three and five years. There is no cure or treatment for this disease.

How Is Tay-Sachs Disease Inherited?

TSD is transmitted from parent to child the same way eye or hair color is inherited. Both the mother and the father must be carriers of the TSD gene to give birth to a child with the disease.

People who carry the gene for TSD are entirely unaffected and usually are unaware that they have the potential to pass this disease to their offspring. When only one parent is a carrier, the couple will not have a child with TSD. When both parents carry the recessive TSD gene, they have a one in four chance in every pregnancy of having a child with the disease. They also have a 50 percent chance of bearing a child who is a carrier. Prenatal diagnosis early in pregnancy can predict if the unborn child has TSD. If the fetus has the disease, the couple may choose to terminate the pregnancy.

Who Is at Risk?

Some genetic diseases, such as TSD, occur most frequently in a specific population. Individuals of Eastern European (Ashkenazi) Jewish descent have the highest risk of being carriers of TSD. According to the National Tay-Sachs and Allied Diseases Association, approximately one in every twenty-seven Jews in the United States is a carrier of the TSD gene, and 85 percent of the children who are victims of this disease are Jewish. Italians also have a higher than average risk of being carriers. In the general population, the carrier rate is one in 250.