Which among the following determines the sex of the child?

The correct answer is Option (1) → Sex chromosomes of the father.

In humans, a female is XX and a male is XY. So every ovum carries an X chromosome. Half the sperms carry X and half carry Y. The sex of the child therefore depends on which sperm fertilises the ovum.

Option (1) Sex chromosomes of the father. — Correct. An ovum fertilised by an X-bearing sperm gives an XX zygote (girl). One fertilised by a Y-bearing sperm gives an XY zygote (boy). Since only the father makes two kinds of gametes, his sex chromosome decides the sex.

Option (2) Sex chromosomes of the mother. — Incorrect. The mother gives an X chromosome to every child. She cannot decide the sex, because all her ova are alike.

Option (3) Duration of mating — Incorrect. The sex is fixed by the chromosomes at fertilisation. The duration of mating has no effect on it.

NCERT Extract:
Fertilisation and Implantation – “… As you know the chromosome pattern in the human female is XX and that in the male is XY. Therefore, all the haploid gametes (ova) produced by the female have the sex chromosome X whereas in the male gametes (sperms) the sex chromosome could be either X or Y, hence, 50 per cent of sperms carry the X chromosome while the other 50 per cent carry the Y. After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilised the ovum. … That is why, scientifically it is correct to say that the sex of the baby is determined by the father and not by the mother!”