What causes Down syndrome?
Solution
Down syndrome is caused by an extra copy of chromosome 21. Here's a step-by-step explanation:
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Every cell in the human body contains genes that are grouped along chromosomes. In most cells, there are 46 chromosomes, grouped into 23 pairs.
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One chromosome from each pair comes from the mother, and the other comes from the father, so we all have a mix of our parents' genes.
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Down syndrome is caused when there's an error in cell division, resulting in an extra copy of chromosome 21. This can occur in one of three ways: Trisomy 21, Mosaicism, or Translocation.
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Trisomy 21 is the most common cause of Down syndrome and occurs when there's an extra copy of chromosome 21 in every cell. This is caused by an error in cell division called nondisjunction, which leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception.
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Mosaicism is when there's a mixture of two types of cells, some containing 46 chromosomes and some containing 47. This happens when the error in cell division occurs after fertilization.
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Translocation occurs when a part of chromosome 21 breaks off during cell division and attaches to another chromosome. The resulting cells have the usual number of chromosomes, but carry extra genetic material from chromosome 21.
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The extra genetic material disrupts the normal course of development, causing the characteristics associated with Down syndrome.
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