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What chromosomal abnormality is associated with Down syndrome?

1. Trisomy 18

2. Monosomy 21

3. Trisomy 21

4. Chromosome X duplication

The correct answer is: Trisomy 21

Down syndrome is primarily associated with Trisomy 21, which means there are three copies of chromosome 21 instead of the usual two. This genetic condition arises during cell division, where an error leads to an extra chromosome being present. The additional genetic material disrupts normal development and is responsible for the characteristic features and health issues seen in individuals with Down syndrome. In contrast, Trisomy 18, Monosomy 21, and Chromosome X duplication do not cause Down syndrome. Trisomy 18 is linked to a different genetic disorder known as Edwards syndrome. Monosomy 21 would imply the absence of a chromosome 21, leading to severe developmental issues that do not coincide with Down syndrome. Chromosome X duplication affects sex chromosomes and does not relate to the patterns of Down syndrome. Thus, Trisomy 21 is unequivocally the chromosomal abnormality connected with Down syndrome.