Tay-Sachs disease is caused by a lack of the hexosaminidase A enzyme, which helps break down gangliosides in nerve cells. This causes gangliosides like GM2 to accumulate in nerve cells, especially in the brain. It is inherited when both parents carry the defective gene, giving their child a 25% risk. Symptoms usually appear within the first 6 months of life and include blindness, loss of motor skills, and seizures. Children with Tay-Sachs disease sadly do not survive past age 4 or 5.