Which of the following answer choices best explains a mechanism as to how Down syndrome might occur?
Choose 1 answer.
Homologous chromosomes fail to separate at chromosome 21 during anaphase II in a parent's
gamete.
Sister chromatids fail to separate at chromosome 21 during anaphase II in a parent's gamete.
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Crossing over doesn't occur during prophase I in a parent's gamete.
Sister chromatids fail to separate at chromosome 21 during anaphase I in a parent's gamete.