SAT Biology Subject Test Practice Questions

Answer: C - Chromosomes normally exist in pairs, having an extra chromosome is trisomy.

Key Takeaway: Disomy, or chromosomes that exist in pairs, is the normal chromosomal pattern. Monosomy is the condition of missing a chromosome, and trisomy is the condition of having an extra chromosome. Trisomy of the 21th chromosomal pair results in Down’s syndrome. Tetrasomy and pentasomy refer to having four and five copies of a chromosome, respectively. Both generally occur only with the sex chromosomes.