The determining chromosome is the X chromosome, not the Y. When the sperm and egg come together, they form the zygote. Reproductive cells each have half the total number of chromosomes in all of the other cells. Each human love cell has 23, whereas most others in the body have 46. Sperm either carry an X chromosome set or a Y chromosome set. If it's an X, the fetus will be female. If it's a Y, it will be male.