In birds, which sex chromosomes determine sex, and which is female vs male?

• A. Female: ZX; Male: ZZ.
• B. Female: ZZ; Male: ZX.
• C. Female: XX; Male: XY.
• D. Female: XY; Male: XX.

Answer: (A)

In birds, sex is determined by a ZW system. The sex chromosomes are Z and W. The female is the heterogametic sex, carrying Z and W (ZW), and the male is homogametic, carrying two Z chromosomes (ZZ). This means females lay eggs that can contribute either a Z or a W, while males contribute only Z.

So the correct pattern is female ZW and male ZZ. The option that reflects this pattern matches the expected biology, though it appears to have a small typographical error in the female chromosome symbol (ZX is meant to be ZW).