Final answer:
In humans, the chance of a sperm carrying an X chromosome is 50%, so approximately 50% of human sperm cells carry an X chromosome.
Step-by-step explanation:
In humans, the sex of a baby is determined by the father's sperm. If the father's sperm carries a Y chromosome, the baby will be male. If the father's sperm carries an X chromosome, the baby will be female. The chance of a sperm carrying an X chromosome is 50%, so approximately 50% of human sperm cells carry an X chromosome.