Sex Determination
Read the following passage carefully and answer the given questions.
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 determining mechanism in case of humans is XY type. A pair of X-chromosomes are present in the female, whereas the presence of an X and Y chromosome are determinant of the male characteristic. 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. The sex determination in honey bee is based on the number of sets of chromosomes which is an individual receives. An offspring formed from the union of a sperm and an egg develops as a female (queen or worker), and an unfertilised egg develops as a male (drone) by means of parthenogenesis.
In humans, the genetic makeup of the ________ determines the sex of the child.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Sperm
A male makes two types of sperm in equal numbers, half with X and half with Y. An ovum fertilised by an X sperm gives a girl, and one fertilised by a Y sperm gives a boy. So it is the genetic makeup of the sperm that decides the sex of the child.
(1) Morula — Incorrect. This is an early embryo of 8 to 16 cells. The sex has already been fixed by then.
(2) Sperm — Correct. It carries either X or Y.
(3) Ovum — Incorrect. Every ovum carries an X chromosome, so it cannot decide the sex.
(4) Zygote — Incorrect. The zygote has the sex chromosomes after fertilisation. But which sex it will be depends on which sperm fertilised the ovum.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
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.”