What does the term trisomy describe?

• A. Three copies of a chromosome
• B. One copy of a chromosome
• C. No copies
• D. Extra sets of chromosomes

Answer: (A)

Trisomy describes having an extra copy of a single chromosome, so that chromosome appears three times in a cell instead of the normal two. In humans, a diploid cell normally has two copies of each chromosome; when nondisjunction during meiosis or mitosis occurs, an gamete can carry an extra chromosome, and fertilization yields a zygote with 2n+1 for that chromosome. This is different from monosomy (one copy), nullisomy (no copies), or polyploidy (whole extra sets of chromosomes). A well-known example is trisomy 21, which leads to Down syndrome.