There are many factors that are important for how the body develops. These include genetics and hormones. In terms of genetics, most boys have one X and one Y chromosome. Most girls have two X chromosomes. However, some boys and girls have different chromosome patterns. For example, some girls have a Y chromosome.
In terms of hormones, two important hormones are testosterone and estrogen. These are made by glands called gonads. There are two types of gonads: testes and ovaries. Testes make testosterone. Ovaries make estrogen and some testosterone. Before birth, testosterone usually makes the body develop as a boy. People whose bodies do not make large amounts of testosterone develop as a girl. Some people’s bodies make testosterone but do not respond to it. Their bodies also develop as a girl. Later, hormones make the body change at puberty.
How a person’s body grows and develops (their “sex”) is not the same as their “gender”. “Gender” is about a person’s sense of self. It is about their inner feeling of being a boy/man or girl/woman, or both, or neither.
Learn more about Gender and Sex: What Makes a Person a Boy or a Girl?