Why is it believed that the father determines the sex of his children?

Answer 1

In humans, the sex of an individual is determined by what combination of sex chromsomes he or she possesses. The sex chromsomes are called 'X' and 'Y' (because of their shape). Females possess two copies of the X chromosome, while males have one X and one Y. In more technical language, in humans the female is the homogametic sex and the male is the heterogameticsex. These terms give a clue to the answer to your question. Females can only produce one type of egg, those with an X chromosome, since they only have one type of sex chromosome (the X) to work with. Males, on the other hand, can produce two kinds of sperm: those carrying an X chromosome, and those carrying a Y. Since every human--male or female--carries at least one X chromosome, and the female can only produce gametes with X chromosomes, the male determines the sex of the child, because if he contributes another X chromosome, the child will be female, and if he contributres a Y, the child will be male. Note: in some other organisms, such as birds, the situation is reversed: the female is the heterogametic sex, and the male the homogametic sex. So in birds, the female determines the sex of the offspring.

Answer 2

The sex of a child is determined by the male's sperm carrying either an X or Y chromosome, which combines with the female's X chromosome. This belief stems from the understanding of biological processes of fertilization, where the sperm carrying a Y chromosome will result in a male child, while the sperm carrying an X chromosome will result in a female child.