Tay-Sachs disease is a rare disorder passed from parents to child. In the most common form, a baby about 6 months old will begin to show symptoms. Tay-Sachs disease results when an enzyme that helps break down fatty substances is absent.ay-Sachs disease is a rare disorder passed from parents to child. In the most common form, a baby about 6 months old will begin to show symptoms. Tay-Sachs disease results when an enzyme that helps break down fatty substances is absent.