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

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). In 1991, with the use of a cDNA clone, it was determined the alpha chain gene is located at 15q23-q24(Nakai et al, 1991). All forms of Tay-Sachs disease are caused by mutations in the alpha chain of the enzyme(Navon et al, 1989). The alpha chain of B-hexosaminidase A is about 35 kilobases long and split into 14 exons(Proia and Soravia, 1987).

Another form of Tay-Sachs disease is Adult onset Tay-Sachs [HexA, GLY269SER]. This form of Tay-Sachs is caused by an amino acid substitution in the alpha chain of the B-hexosaminidase A molecule. Glycine is substituted serine at position 269 in the HexA subunit. This is caused by a G to A substitution at the 3-prime end of exon 7 (Navon and Proia, 1989).

In 1986, a study of 2 Ashkenazi and 2 French-Canadian carriers concluded the populations had different mutations. Tay-Sachs disease [HexA 7.6-kbDEL, EX1] is a deletion mutation. 7.6 kilobases are deleted, including part of intron 1 and all of exon 1. The deletion extends 2000 basepairs past the promoter region of the alpha-chain gene (Myerowitz and Hogikyan, 1987).

There are at least thirty different mutations that cause Tay-Sachs disease. A majority of the classical (infantile) form of the disease that is found in the Ashkenazi Jewish population is caused by one of two different gene mutations( Triggs-Raine et al, 1990).

Some topics in this essay:
Tay Britain, Perche France, Tay-Sachs French-Canadian, Ashkenazi Jews, Louisiana Cajun, INS EX11, Adult Late, Navon Proia, IVS< G-C, HexA GLY269SER, tay-sachs disease, et al, alpha chain, heterozygous carriers, disease hexa, tay-sachs disease hexa, ashkenazi jews, onset tay-sachs, al 1992, form tay-sachs, located chromosome, et al 1992, alpha chain b-hexosaminidase, adult onset tay-sachs, al 1992 tay-sachs,