In humans, the sex of the baby is determined by the sperm. The sperm gamete is heterogamatic because approximately half of them contain the X chromosome which will result in a girl and approximately half of them contain the Y chromosome which will result in a boy. On the other hand the ova gamete is homogametic, in other words all its cells possess the X chromosome. Thus the ova gamete does not influence the baby’s sex. Therefore when a sperm penetrates the ova and fertilization occurs there are two possible outcomes: XX or a girl and XY or a boy.
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