Mistake in chromosom: During the formation of normal eggs or sperm the chromosome(c) pairs are supposed to split leaving one of each in the egg/sperm.At conception the joining of egg/sperm brings together the pair & as tissue grows it has the proper number in each cell. With ds the egg has a pair of 21c and at conception a triple 21 is created. All cells derived after will have an extra 21 & become a ds child.
Answered 9/28/2016
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