Identify the incorrect pair related to sex determination in various organisms:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Drosophila = XX-XO type
Sex is decided by the sex chromosomes, in one of three ways in the book’s examples. In the XO type the male has only one X chromosome. In the XY type the male has one X and one Y. In the ZW type the female has two different sex chromosomes (ZW) and the male two similar ones (ZZ).
Option (1) Human = XX-XY type — A correct pair. Human females have XX and males have XY, so humans show the XX-XY type, with the male producing two kinds of sperms.
Option (2) Grasshopper = XO type — A correct pair. Grasshopper males have only one X chromosome besides the autosomes, while females have a pair of X chromosomes. This is the XO type.
Option (3) Drosophila = XX-XO type — The incorrect pair. In Drosophila, as in humans, the males have one X and one Y chromosome. So Drosophila shows the XX-XY type, not XX-XO.
Option (4) Birds = ZZ-ZW type — A correct pair. In birds such as the chicken, the female is ZW and the male ZZ. Here it is the female that produces two kinds of gametes.
NCERT Extract:
Sex Determination – “… Grasshopper is an example of XO type of sex determination in which the males have only one X-chromosome besides the autosomes, whereas females have a pair of X-chromosomes.”
Sex Determination – “… Both males and females bear same number of autosomes. Hence, the males have autosomes plus XY, while female have autosomes plus XX. In human beings and in Drosophila the males have one X and one Y chromosome, whereas females have a pair of X-chromosomes besides autosomes (Figure 4.12 a, b).”
Summary – “… In chicken, sex chromosomes in male are ZZ, and in females are ZW.”