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National Human Genome Research Institute (Human Genome Project) Public domain, via Wikimedia Commons

Humans normally have 46 chromosomes spread out over 23 pairs. Down syndrome, also known as trisomy 21, occurs when there's an extra chromosome at the chromosome 21 pair (shown above).

Which of the following answer choices best explains a mechanism as to how Down syndrome might occur?

Answer: CORRECT (SELECTED)
Sister chromatids fail to separate at chromosome 21 during anaphase II in a parent's gamete.

Explanation:
This failure to separate results in the formation of a gamete that has two chromosomes at spot 21 when they should only have one. When this gamete fuses with the other parent's gamete, a total of three chromosomes are at spot 21 in the offspring.

National Human Genome Research Institute Human Genome Project Public domain via Wikimedia Commons Humans normally have 46 chromosomes spread out over 23 pairs D class=

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luisvaschetto

Down syndrome is due to sister chromatids failing to separate at chromosome 21 during anaphase II in a parent's gamete.

What are sister chromatids?

Sister chromatids are each one of the duplicated genetic material during meiosis that may contain different genomic fragments due to cross-over after prophase I.

Each chromosome contains 2 sister chromatids, thereby diploid cells contain 2 homologous chromosomes and 4 sister chromatids before meiosis II.

In conclusion, Down syndrome is due to sister chromatids failing to separate at chromosome 21 during anaphase II in a parent's gamete.

Learn more about sister chromatids here:

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