Which chromosome pair would a normal female duck have?

• A. ZZ
• B. ZW
• C. XX
• D. XY

Answer: (B)

Birds use a ZW sex-determination system. In this setup, females are heterogametic, meaning they have one Z and one W chromosome, while males are homogametic with two Z chromosomes. So a normal female duck carries a Z and a W. When fertilization occurs, if the egg (which carries either Z or W) is fertilized by a sperm carrying Z, the result is ZZ (a male). If the egg carries W, the result is ZW (a female). Therefore, the female duck is ZW. ZZ would be male in birds, and XX or XY describe mammalian or other species’ systems, not the avian female.