Tay-Sachs disease is a terminal genetic disorder in which harmful quantities of a fatty substance called ganglioside GM2 accumulate in the nerve cells in the brain. Infants with Tay-Sachs disease appear to develop normally for the first few months of life. Then, as nerve cells become distended with fatty material, a relentless deterioration of mental and physical abilities occurs. The child becomes blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in.

A much rarer form of the disorder that occurs in patients in their twenties and early thirties is characterized by unsteadiness of gait and progressive neurological deterioration.

Patients with Tay-Sachs have a "cherry-red" spot in the back of their eyes. The condition is caused by insufficient activity of an enzyme called hexosaminidase A that catalyzes the biodegradation of acidic fatty materials known as gangliosides. Gangliosides are made and biodegraded rapidly in early life as the brain develops. Patients and carriers of Tay-Sachs disease can be identified by a simple blood test that measures hexosaminidase A activity. Both parents must be carriers in order to have an affected child (which makes it a recessive trait on the 15th chromosome). When both parents are found to carry a genetic mutation in hexosaminidase A, there is a theoretical 25 percent chance with each pregnancy that the child will be affected with Tay-Sachs disease. Prenatal monitoring of pregnancies is available.

Presently there is no treatment for Tay-Sachs. Even with the best of care, children with Tay-Sachs disease usually die by age 5.

Notably, this is a familial disorder found in east European Jewish (Ashkenazi) families. Symptoms begin to appear at 3 to 6 months old and progress rapidly, and the child usually dies before 3 years old. The only known risk factor is familial descent. In this group, the rate of fatality/birth with the disorder is 1 out of 2,500 people.

Symptoms include:
  • Blindness
  • Deafness
  • Dementia
  • Listlessness
  • Irritability
  • Seizures
  • Paralysis
  • Decreased muscle tone (loss of muscle strength)
  • Muscle function loss
  • Temper or agitation (exaggerated startle response)
  • Delayed mental and social skills
  • Growth (slow)

source: National Ty-Sachs & Allied Diseases Association, Inc.

Thaks to LadySun for bugging me to make this writeup more accurate. Creds go to her for the Chromosome 15 clarification.