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Trisomy 21 results from

  • A.non-disjunction giving an extra chromosome 21
  • B.the deletion of a part of chromosome 21
  • C.a crossing over event within chromosome 21 itself
  • D.the complete absence of chromosome 21 from the cell

Answer: A

A gamete carrying two copies instead of one produces a zygote with three. Chromosome 21 is small, which is part of why trisomy of it is survivable when most others are not. Trisomies of larger chromosomes are almost always lost early in pregnancy.

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