Health Library
Gender and Sex: What Makes a Person a Boy or a Girl?

Last Updated 06/2022

What Makes a Person a Boy or a Girl?

Both “sex” and “gender” are related to being a boy or a girl. “Sex” is about a person’s body parts and how their body grows and develops. A person’s sex is due to a mix of genes and hormones. You can think about genes as an instruction code and hormones as chemical messages.

In terms of genes, most people have 46 chromosomes. (Chromosomes are large groups of genes.) Two of the chromosomes, the X and the Y, are called sex chromosomes. Most boys have one X and one Y chromosome. Most girls have two X chromosomes. However, some boys and girls may have different chromosome patterns. For example, some girls have one X and one Y chromosome. Changes in some genes may explain why some people’s bodies develop differently.

Hormones are chemical messages that a person’s body makes. They tell the body how to grow and develop. Sex hormones make a person’s body parts develop as a boy or a girl. Sex hormones are made by glands inside the body called gonads. There are two types of gonads: testes and ovaries. Most boys have testes and most girls have ovaries. Testes make hormones called androgens. Testosterone is one type of androgen. Ovaries make estrogen, and some testosterone. In addition, testes make sperm and ovaries make eggs.

In terms of body parts, people have reproductive parts inside and outside their bodies. Gonads are one of the parts inside the body. Also inside the body, most girls have a vagina, a cervix and a uterus. The parts outside the body are called genitals. Most boys have a penis and a scrotum. Most girls have a clitoris, labia and a hole for the vagina. Some children have a mix of male and female reproductive parts, like testes and a vagina, or genitals that are not clearly male or female. This is called a difference or disorder of sex development. Some people may use different words, such as intersex.

Before birth and at puberty, hormones make the body parts grow and change. The growth and change depend on several things. They depend on what hormones the body makes, how much it makes, and whether the body responds to the hormones. In other words, a person’s body has to make enough hormones and be able to respond to the hormones, to grow and develop. Before birth, testosterone makes the body develop as a boy. The body develops a penis and a scrotum. If a person’s body does not make or respond to testosterone, their body develops as a girl. They develop a clitoris, labia and a vagina.

At puberty, people make larger amounts of testosterone and estrogen. These cause more changes in their bodies. Testosterone makes both men and women grow hair in their armpits and groins. (Hair in the groin is called pubic hair.) Testosterone also makes men have stronger muscles, deeper voice, and facial hair. Estrogen makes women have breasts and wider hips. A person’s “sex” is not the same as their “gender.” “Gender” is about a person’s sense of self, and their inner feeling of being a boy / man or girl / woman. (Some people believe that there are more than two genders. Some people also experience their gender as fluid.) When babies are born, they are usually assigned a sex based on how their genitals look. For many people, their gender identity is the same as the sex assigned at birth. However, for some people, their gender identity does not match their assigned sex.

Contact Us.