Explain how, sex is determined in human babies.

The sex of a child depends on what happens at fertilization:

(a) If a sperm carrying X chromosome fertilizes an ovum (or egg) which carries X chromosome, then the child born will be a girl. This is because the child will have XX combination of sex chromosomes.

(b) If a sperm carrying Y chromosome fertilizes an ovum (or egg) which carries X chromosome, then the child born will be a boy. This is because the child will have XY combination of sex chromosomes.