In birds, female sex chromosomes are typically designated as which pairing?

• A. ZZ
• B. ZW
• C. XX
• D. XY

Answer: (B)

Birds use a ZW sex-determination system, meaning the female is heterogametic and carries two different sex chromosomes (Z and W) while the male is homogametic with two Z chromosomes. Because a female egg can contribute either a Z or a W, offspring can be ZZ (typically male) or ZW (female). Therefore, the female pairing is ZW. This contrasts with mammals, where females are XX and males XY, and with birds where the male pairing is ZZ.