Karyotypes can be studied to determine an organism’s chromosomal makeup and to detect genetic defects. Down syndrome is a genetic disorder that is also called trisomy 21. What would trisomy 21 look like in a karyotype?

“Trisomy” means having three copies of a chromosome instead of the usual two. Trisomy 21 therefore appears as an extra chromosome 21 in the karyotype, making “extra chromosome 21” the correct choice.

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