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Tay-Sachs Disease (TSD) is caused by an absence of the enzyme hexosaminidase A which leads to the build up of a substance in cells, especially in the brain, causing mental and physical deterioration. Children with TSD appear healthy at birth, and then gradually lose the ability to perform basic movements, swallow food and are prone to seizures. By the time a child with TSD is three or four years old, the nervous system is damaged and these children rarely survive beyond 5 years of age. The carrier rate in the Ashkenazi Jewish population is approximately 1 in 30 (much higher than in the general population). Higher carrier rates have also been identified in certain French Canadian and Acadian populations.

Canavan Disease (CD) is caused by the lack of a vital enzyme called aspartoacylase. This leads to a defect in myelin, which is needed to protect nerves and allow messages to reach the brain. Although, like TSD, a newborn with CD will first appear healthy, after roughly nine months of age, progressive deterioration begins and few children live beyond infancy or early childhood. Symptoms include a slowing and eventual regression of development including movement skills, inability to speak or swallow, respiratory problems and loss of vision. The carrier rate in the Ashkenazi population is about 1 in 60 and although CD does occur in other ethnic groups, carrier frequency in those groups has not yet been determined.

Familial Dysautonomia (FD) is also known as Riley-Day Syndrome and primarily affects the autonomic and sensory nervous system interfering with a variety of bodily functions. Newborns generally show early evidence of ill health with numerous symptoms including compromised digestion, blood pressure and muscle tone as well as decreased reaction to pain, high fevers and pneumonia. Life expectancy may be greatly reduced. Recently the gene for FD and the two common mutations causing the disease in the Ashkenazi Jewish population were identified. Recent studies show that 1 in 30 Ashkenazi Jews carry the gene for FD and almost all reported individuals with this disease have occurred in this particular population.
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