For the human animal, who has more influence in determining the sex of a child? The mother, the father, or both equally?

When it comes to the genetics of a baby, the biological sex is determined by a pair of chromosomes. A female has two X chromosomes (XX), while a male has one X and one Y chromosome (XY). The mother's egg cell will always contribute an X chromosome, as that is the only type she possesses. Her genetic input into the child's sex is therefore a constant.

The deciding factor comes from the father's sperm, which is where the variation occurs. Roughly half of his sperm carry an X chromosome, and the other half carry a Y chromosome. It is a 50/50 chance which type of sperm will be the one to fertilize the egg. If an X-carrying sperm fertilizes the egg, the resulting XX combination produces a girl. If a Y-carrying sperm is successful, the XY combination produces a boy.

This modern understanding of genetics provides a fascinating contrast to history. For centuries, powerful figures like England's King Henry VIII famously blamed their wives for not producing a male heir. We now know that the genetic potential for a son can only come from the father, as he is the only parent who carries the Y chromosome that triggers male development.