Tay-Sachs disease is a recessive genetic disorder that is common amongst Ashkenazi Jews. It appears in the early months of life. It features a progressive degeneration of all brain functions.
In the general population, one person in 35,000 is a carrier of the gene for Tay-Sach's disease. Among Ashkenazi Jews, 1 person in 30 is a carrier, and about 1 in 2,000 people will suffer from the disease.
Tay-Sach's disease leads to blindness, deafness, progressive dementia, seizures, paralysis and death, usually before the age of three years.
The condition is due to the absence of an essential enzyme that breaks down a substance called ganglioside, present mainly in the nervous system. It is the accumulation of this material that is so damaging.
A recessive genetic disorder is one in which both father and mother must carry the gene before the child can develop the disease. Chromosomes come in pairs, each member of a pair having an almost identical set of genes.
Parents carrying the mutated gene can only have it on one of the gene pairs, the other gene being normal. When sperms and eggs are formed, only one of each chromosome pair enters the sperm or egg.
So it is pure chance whether the sperm and the egg carry the mutated gene or the normal one. For the disease to occur, a sperm carrying the mutation (50:50 chance) must fertilise an egg carrying the mutation (50:50 chance).
The diagnosis is based on the family history, the signs and symptoms and the appearance of a cherry-red spot in the centre of the retina at the back of each eye (the macula).
Unfortunately, there is no treatment for Tay-Sachs disease, so it is very important for carriers or possible carriers to have genetic counselling. Carrier status can be found by a blood test.
Antenatal diagnosis (testing before birth) by Chorionic villus sampling can be done. If the diagnosis is confirmed, termination of pregnancy can be offered.
Some Jewish communities have successfully organised pre-marital counselling services and testing in the hope of preventing the occurrence of this disorder. As a result, the incidence of Tay-Sachs disease dropped by 80 per cent between 1970 and 1980.