Specifically, it is caused by a mutation in the HEXA gene located on chromosome 15. This gene provides instructions for making an enzyme called hexosaminidase A, which is crucial for breaking down a fatty substance called GM2 ganglioside in the brain and nerve cells.

When the HEXA gene is mutated, the body cannot produce enough hexosaminidase A, leading to a build-up of GM2 ganglioside in the brain. This accumulation causes progressive damage to nerve cells, leading to the symptoms of Tay-Sachs disease.