CBSE Class 8 Answered

The sex of the baby is determined by the type of sex chromosome present in the fertilised egg from
which the baby develops.

 

More information is as follows: - 

The females carry two X-chromosomes. Females produce one type of gametes (eggs) with same type of chromosomes (22 + X). Males have one X and one Y chromosome. Among the male gametes, half of the sperms carry X-chromosome (22 + X) and half carry Y-chromosome (22 + Y). Thus female is homogametic and male is heterogametic.

When a sperm carrying X chromosome fertilizes an egg, the zygote develops into female (XX condition). When sperm carrying Y chromosome fertilizes an egg, the zygote develops into a male (XY condition). Thus, sex is determined at the time of fertilization.