In humans, the determination of a child’s sex is based on the combination of sex chromosomes inherited from both parents. However, the father’s contribution is crucial in determining the sex of the child due to the nature of sex chromosomes.
The father contributes both an X and a Y chromosome in his sperm. Meanwhile, the mother provides one X chromosome in her egg. It’s the sperm’s chromosome that determines the sex of the child. If the sperm carries an X chromosome, the resulting combination (XX) will develop into a female, while if the sperm carries a Y chromosome, the combination (XY) will develop into a male.
Therefore, it’s the father’s sperm, specifically whether it carries an X or a Y chromosome, that ultimately determines the sex of the child, making the father responsible for the sex determination process.