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are medical conditions that are caused by an error in a person's genes. Some of them appear as a birth defect, while others do not become distinct until later in life. can range from those that cause death to those that produce only mild problems, such as col ....

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.... movements and may have problems with coordination, thinking, and judgment. A weakening of the nerve cells in the brain causes these symptoms and later results in death. Another single-gene disorder is the Tay Sachs Disease. People who inherit the faulty Tay Sachs gene lack a crucial enzyme that is needed to break down fatty substances in brain and nerve cells. As a result, these substances build up in such large quantities that the central nervous system gradually stops functioning and the person dies. Symptoms usually become evident within the first six months of life, and most children die before reaching the age of four. 1 i ....