Read the passage and answer the following questions:
The initial clue about the genetic/chromosomal mechanism of sex determination can be traced back to some of the experiments carried out in insects. Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few insects, and it was also observed by him that 50 per cent of the sperm received this structure after spermatogenesis, whereas the other 50 per cent sperm did not receive it. Henking gave a name to this structure as the X body but he could not explain its significance. Further investigations by other scientists led to the conclusion that the 'X body' of Henking was in fact a chromosome and that is why it was given the name X-chromosome. Due to the involvement of the X-chromosome in the determination of sex, it was designated to be the sex chromosome, and the rest of the chromosomes were named as autosomes.
Given below are two statements:
Statement I: In grasshopper, the eggs fertilised by sperm having an X-chromosome become females.
Statement II: In grasshopper, the males and the females have the same number of chromosomes.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Statement I is true but Statement II is false
Grasshopper shows the XO type of sex determination. All eggs carry an X-chromosome besides the autosomes. Some sperms carry an X-chromosome and some do not. The sperm that fertilises the egg decides the sex.
Statement I: In grasshopper, the eggs fertilised by sperm having an X-chromosome become females. — True. An egg (X) fertilised by a sperm with an X-chromosome gives two X-chromosomes, so the offspring becomes a female. A sperm without an X gives a male (X0).
Statement II: In grasshopper, the males and the females have the same number of chromosomes. — False. Both sexes have the same autosomes, but the male has only one X-chromosome and the female has a pair. So the male has one chromosome fewer than the female. Equal numbers in both sexes are seen in the XY type, as in humans.
NCERT Extract:
Sex Determination – “… It was also observed that in a large number of insects the mechanism of sex determination is of the XO type, i.e., all eggs bear an additional X-chromosome besides the other chromosomes (autosomes). On the other hand, some of the sperms bear the X-chromosome whereas some do not. Eggs fertilised by sperm having an X-chromosome become females and, those fertilised by sperms that do not have an X-chromosome become males. … 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.”