Tay-Sachs disease is a fatal, genetic disorder of the nervous system. Tay-Sachs was first identified in the 1880's by two physicians. Dr. Bernard Sachs of the United States has found a "cherry-red" spot in the eyes of a patient. That patient later died. After searching medical literature, he found Warren Tay of great Britain had also reported this (Information, 1994).
The symptoms of Tay-Sachs disease appear after about six months. At first, the patient has an over-exaggerated "startled" reaction to sounds and begins to loose control of its head. Eventually, it cannot roll over or sit without help. Dementia (uncontrolled laughter) may set in and the head grows abnormally large. The baby then becomes blind, and dies, usually before its 5th year (Seely et al, 1992). .
Tay-Sachs disease is an autosomal, recessive disorder caused by a deficiency in B-hexosaminidase A. Being an autosomal recessive disease, Tay-Sachs can only be passed on in its fatal form if both parents are heterozygous for the disease. If both parents are heterozygous for Tay-Sachs, there is a one in four chance of the infant having the disease. If only one parent is heterozygous, the infant has a one in two chance of being a carrier (heterozygous) for the disease(Mahany et al, 1994). In 1962, researchers found B-hexosaminidase A is responsible for the breakdown of ganglioside (gm2) in nerve cells. Ganglioside is a lipid found in modest levels in nerve cell membranes. It is constantly being synthesized and broken down. Without the B-hexosaminidase A to break down the gm2, the cells swell up and eventually burst( Diamond, 1991).
B-hexosaminidase A is composed of two amino acid chains, the alpha and the beta chain(Navon et al, 1989). The gene responsible for the manufacture of B-hexosaminidase A was originally thought to be located on chromosome 7(Gilbert et al, 1975). It was later determined that the gene for the alpha chain is located on chromosome 15, and the beta chain gene is located on chromosome 5( Chern et al, 1976).