Down's Syndrome
Definition
Down's syndrome is the most common cause of mental retardation (see mental retardation entry) and malformation in newborns. It occurs because of the presence of an extra chromosome.
Description
Chromosomes are structures that carry genetic information in cells. They carry the instructions that tell cells what functions they are to perform. They determine the way a person's body looks and the way it functions.
Cells normally carry two matched sets of twenty-three chromosomes for a total of forty-six chromosomes. One set of twenty-three chromosomes comes from each parent. Down's syndrome occurs when one chromosome pair is damaged. That pair is designated as chromosome #21.
Causes
Chromosome #21 can be damaged, for example, if one parent has two chromosomes at location 21, rather than one. A child will then get two #21 chromosomes from one parent and one #21 chromosome from the other parent, for a total of three #21 chromosomes. This form of Down's syndrome is called trisomy 21, meaning that the #21 chromosome has three units rather than two. Overall, the child has forty-seven chromosomes, rather than forty-six chromosomes. This pattern of changes occurs in more than 90 percent of all Down's patients.
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