Humans have two different types of sex chromosomes and hence, said to have heterogamy. Human male has chromosome XY and female has chromosome XX. That means, humans male have two types of gametes that is, X and Y and female has gametes with only X chromosome.

Now, if male gamete is having chromosome X fuses with ovum having chromosome X, this results into a female child.

And, if male gamete is having chromosome Y fuses with ovum having chromosome X, this results into a male child.