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: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In human beings, it is the genetic makeup of the sperm that determines the sex of the child.
Reason (R): In human beings, there is a 50 per cent probability of either a male or a female child in each pregnancy.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): In human beings, it is the genetic makeup of the sperm that determines the sex of the child. — True. A is about which parent’s gamete decides the sex. In humans the female makes only one type of ovum, with an X-chromosome. The male makes two types of sperm: half carry an X and half carry a Y. An ovum fertilised by an X-sperm gives a girl (XX); by a Y-sperm, a boy (XY). So the sperm decides.
Reason (R): In human beings, there is a 50 per cent probability of either a male or a female child in each pregnancy. — True. R is about the chance of a boy or a girl in each pregnancy. X-sperm and Y-sperm are made in equal numbers and have an equal chance of fertilising the ovum, so each pregnancy has a 50 per cent chance of a male or a female child.
Why R does not explain A? A and R are two separate results of the same fact: the male makes two kinds of sperm and the female makes one kind of ovum. A follows from which gamete comes in two kinds. R only states the probability of each sex. A 50 per cent chance by itself does not show that the sperm is the deciding gamete, so R is not the correct explanation of A.
NCERT Extract:
Sex Determination in Humans – “… During spermatogenesis among males, two types of gametes are produced. 50 per cent of the total sperm produced carry the X-chromosome and the rest 50 per cent has Y-chromosome besides the autosomes. Females, however, produce only one type of ovum with an X-chromosome. There is an equal probability of fertilisation of the ovum with the sperm carrying either X or Y chromosome. In case the ovum fertilises with a sperm carrying X-chromosome the zygote develops into a female (XX) and the fertilisation of ovum with Y-chromosome carrying sperm results into a male offspring. Thus, it is evident that it is the genetic makeup of the sperm that determines the sex of the child. It is also evident that in each pregnancy there is always 50 per cent probability of either a male or a female child.”