The Body as a Whole Practice Test

The main idea is that Down syndrome is caused by trisomy 21, meaning there are three copies of chromosome 21. In humans, normal somatic cells have 46 chromosomes—22 autosome pairs plus 1 pair of sex chromosomes. Adding one extra chromosome 21 raises the total to 47 chromosomes in the cells. So the correct total is 47. The other numbers would reflect different chromosomal issues: 45 would mean a missing chromosome, 46 is normal, and 48 would indicate two extra chromosomes. In classic trisomy 21, most cells carry 47 chromosomes.