Question

Which of the following is incorrect regarding the 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 offsprings
• D. All of these

Answer 1

The correct option is B Females are homogametic and males are heterogametic

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.


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).

Finding out the possible genotypes by Punnett square
$\begin{array}{cc}\text{Gametes}& \text{Z(male gamete)}\\ \text{Z (female gamete)}& \text{ZZ (male)}\\ \text{W (female gamete)}& \text{ZW(female)}\end{array}$
The ratio of male(ZZ) and female(ZW) is found to be 1:1.

This mechanism operates in some vertebrates (fishes, reptiles, and birds).
As we have been asked to find the incorrect option, it should be option (b): 'Females are homogametic and males are heterogametic'.