Question

Which of the following is incorrect regarding $ZW$-$ZZ$ type of sex determination ?

• A. It occurs in birds and some reptiles
• B. Females are homogametic and males are heterogametic
• C. $1:1$ sex ratio is produced in the offsprings
• D. All of these

Answer 1

Answer: (B)

In $ZW$ - $ZZ$ type of sex determination, the male has two homomorphic sex chromosomes $(ZZ)$ and is homogametic, and the female has two heteromorphic sex chromosomes $(ZW)$ and is heterogametic. There are, thus, two types of eggs : with $Z$ and with $W$, and only one type of sperms, i.e., each with $Z$. Fertilisation of an egg with $Z$ chromosome by a sperm with $Z$ chromosome gives a zygote with $ZZ$ chromosomes (male). Fertilisation of an egg with $W$ chromosome by a sperm with $Z$ chromosome yields a zygote with $ZW$ chromosomes (female). This mechanism operates in some vertebrates (fishes, reptiles and birds).