What is the chromosomal defect that underlies Down syndrome?

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Down syndrome is caused by the presence of a third copy of chromosome 21, also known as trisomy 21. It occurs due to a mistake in cell division during the production of gametes (eggs and sperm) or during early embryonic development. This results in individuals with three copies of chromosome 21 instead of the typical two, leading to intellectual disability and a range of physical features.

What Is the Chromosomal Defect That Underlies Down Syndrome