Down's syndrome is condition with several symptoms, including a characteristic body type, mental retardation, increased susceptibility to infections, and various heart and other organ abnormalities. It is a result of an extra copy of chromosome 21. In most cases this is a result of one of the parents' gametes not dividing properly. In some cases, it is a result of one of the parents having a chromosomal abnormality in which chromosome 14 and 21 are merged. This can be detected by looking at the parents' chromosomes.
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