tay-sachs is a genetic disease. it happens when chromosome 15 has a genetic problem. the problem is caused by a defective hexa gene. it is recessive, which means both parents must give the baby the defective gene for the baby to have tay-sachs. one in every twenty seven jewish american has one defective gene and can give tay-sachs the defective gene to their child. the disease is very rare. it is most common in jewish families. there is a blood test to see if a person has the tay sachs gene. this blood test has helped reduce the number of babies born with tay-sachs. in the 1800’s, there were 60 new cases and in the 21st century there were 5.