Sex in human embryo is determined by:

The correct answer is Option (2) → ‘X’ or ‘Y’ chromosome of sperm

Option (1) ‘X’ chromosome of egg — Incorrect. Every egg carries the same X chromosome. Since the egg gives no choice, it cannot decide the sex.

Option (2) ‘X’ or ‘Y’ chromosome of sperm — Correct. The female makes only X eggs, while the male makes X and Y sperms in equal numbers. So the sperm decides the sex: an X sperm gives a girl (XX) and a Y sperm a boy (XY).

Parents: mother XX × father XY
Eggs: all X    Sperms: 50% X, 50% Y
X egg + X sperm → XX, a girl
X egg + Y sperm → XY, a boy

So in each pregnancy there is a 50 per cent chance of either a boy or a girl.

Option (3) Only ‘Y’ chromosome of sperm — Incorrect. A Y-carrying sperm gives a boy, but a girl results from an X-carrying sperm. Both kinds of sperm decide the sex, not the Y alone.

Option (4) Health of mother — Incorrect. The mother’s health has no effect on the sex chromosomes of the child. Blaming women for the birth of girls is a false notion.

NCERT Extract:
Sex Determination in Humans – “It has already been mentioned that the sex determining mechanism in case of humans is XY type. Out of 23 pairs of chromosomes present, 22 pairs are exactly same in both males and females; these are the autosomes. 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. 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.”