Given below are two statements:
Statement I: The ZW-ZZ type of sex determination occurs in many birds.
Statement II: In the ZW-ZZ type of sex determination, females are heterogametic and males are homogametic.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Both Statement I and Statement II are correct
Statement I: The ZW-ZZ type of sex determination occurs in many birds. — True. Birds show a sex-determining mechanism different from humans. The book’s figure on sex determination states that in many birds the female has ZW and the male has ZZ sex chromosomes.
Statement II: In the ZW-ZZ type of sex determination, females are heterogametic and males are homogametic. — True. A female bird has one Z and one W chromosome, so she makes two kinds of eggs (Z or W). This is female heterogamety. A male bird has two Z chromosomes and makes only one kind of sperm, so he is homogametic. This is the reverse of the XY type, where the male is heterogametic.
NCERT Extract:
Sex Determination – “… In some other organisms, e.g., birds, a different mechanism of sex determination is observed (Figure 4.12 c). In this case the total number of chromosome is same in both males and females. But two different types of gametes in terms of the sex chromosomes, are produced by females, i.e., female heterogamety. In order to have a distinction with the mechanism of sex determination described earlier, the two different sex chromosomes of a female bird has been designated to be the Z and W chromosomes. In these organisms the females have one Z and one W chromosome, whereas males have a pair of Z-chromosomes besides the autosomes.”