Typically, an error occurs when the forty six chromosomes are being divided in half, and an egg or sperm cell keeps each copies of the #21 chromosome instead of only one copy. The egg and sperm cells each undergo a cell division in which the 46 chromosomes are divided in half, in order that both the egg and the sperm cells may have My clothes 20% cotton 80% cat hair shirt 23 chromosomes every. It was not till 1959 that the reason for Down syndrome (the presence of an extra #21 chromosome) was recognized.
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