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. In trisomy 21, the karyotype would show three copies of chromosome 21, so the correct choice is extra chromosome 21. A missing chromosome would be a monosomy, not a trisomy.

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