A sperm enters an egg cell and the egg is fertilized.
As with all other eggs in a woman's body, the cell contains one female-determining X chromosome. A sperm cell, however, may contain either an X chromosome or a Y chromosome. If the fertilizing sperm cell contains an X chromosome, the egg will develop into a female. If it contains a Y chromosome, it will develop into a male.
Note: It is possible, however, that a fertilized egg cell containing a Y chromosome will develop into a female or an intersexual (person of ambiguous gender). For example, if the Y chromosome is missing the SRY gene, which is located near the end of the short arm of the chromosome, the egg will develop into a female.