Definition of Tay-Sachs disease:

Tay-Sachs disease is a deadly disease of the nervous system passed down through families.

Causes, incidence, and risk factors:

Tay-Sachs disease occurs when the body lacks hexosaminidase A, a protein that helps break down a chemical found in nerve tissue called gangliosides. Without this protein, gangliosides, particularly ganglioside GM2, build up in cells, especially nerve cells in the brain.

Tay-Sachs disease is caused by a defective gene on chromosome 15. When both parents carry the defective Tay-Sachs gene, a child has a 25% chance of developing the disease. The child must receive two copies of the defective gene -- one from each parent -- in order to become sick. If only one parent passes the defective gene to the child, the child is called a carrier. He or she won't be sick, but will have the potential to pass the disease to his or her own children.

Anyone can be a carrier of Tay-Sachs, but the disease is most common among the Ashkenazi Jewish population. About 1 in every 27 members of the Ashkenazi Jewish population carries the Tay-Sachs gene.

Tay-Sachs has been classified into infantile, juvenile, and adult forms, depending on the symptoms and when they first appear. Most people with Tay-Sachs have the infantile form. In this form, the nerve damage usually begins while the baby is still in the womb. Symptoms usually appear when the child is 3 to 6 months old. The disease tends to get worse very quickly, and the child usually dies by age 4 or 5.

Late-onset Tay-Sachs disease, which affects adults, is very rare.