Answer:
In a diploid zygote, the cause of trisomy is two sister chromatids remain joined during meiosis II (option B).
Explanation:
At the end of meiosis I, each daughter cell contains a chromosome formed by sister chromatids, each of which must be separated in meiosis II to form the gametes with a chromosome charge equivalent to half the chromosome charge of the species.
The result of the sister chromatids not separating in meiosis II -by not disjunction- results in a gamete with an extra chromosome, which when joined to another gamete results in a zygote with an additional chromosome, which is considered an aneuploidy, called trisomy.
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