At what point is sex determined in the reproductive process?

Sex determination occurs at fertilization when the sperm and egg combine. This event leads to the formation of a zygote. The genetic material contributed by each gamete carries sex chromosomes; typically, the egg contributes an X chromosome, while the sperm can contribute either an X or a Y chromosome. The combination of these chromosomes establishes the genetic sex of the zygote: XX typically corresponds to a female and XY to a male.

While other stages like meiosis and mitosis play crucial roles in the overall reproductive process—meiosis in producing gametes and mitosis in cell division and growth—they do not directly determine the sex of the organism. The genetic information that defines sex comes into play specifically at the moment of fertilization, making it the critical point for determining the future organism's sex.