Whether to give birth to a boy or a girl is determined by sperm. The gene of an egg cell is represented by X, and the gene of a sperm can be represented by X and Y. If sperm X combines with an egg cell, it will give birth to a girl, while sperm Y combines with an egg cell to give birth to a boy.
If two sperm combine with an egg cell, X and Y, identical twins will be born; if two sperm combine with two eggs, fraternal twins will be born.