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Biology and transgender people

We do not know for sure why some people are transgender. Some scientists are looking for the answers. This is what they know right now.

This page uses easy words. This helps young people read it. This also helps people who do not know many English words. The words in bold are hard. You need to know what they mean, or this will be hard to read. You can use these links to look up words:

Merriam-Webster’s Learner’s Dictionary

Cambridge Learner’s Dictionary

human who works in science is called a scientist. Some of them study life. The study of life is biology. A person who studies biology is a biologist. Some biologists study reproduction. Reproduction is how living things make more living things.

The science of reproduction is going to change a lot by the end of the 21st century. That is why some people are scared of transgender people. They are often scared about what will happen when we can change humans and make new ones using new science.

How are human babies made?

Sexually mature humans can often make more humans by sexual reproduction. They can often make a gamete inside of their bodies. A gamete is a special kind of cell that is also called a haploid. Haploid is from the Greek word for single, because a haploid can carry half of the genes needed to make a human. At this time, there are two kinds of human gametes: eggs and sperm. Almost all humans born so far are made from one egg and one sperm.

Most human babies have nine months of development from being made to being born. This development time is called gestation. At first, a sperm can join an egg. This is called fertilization. After fertilization, the joined egg and sperm haploids form a diploid cell called a zygote. This cell joins the genes from two people inside one cell. When the zygote starts to grow one day later, it’s called an embryo. The embryo grows for 8 weeks. Then it’s called a fetus until it is born. After it’s born, it’s called a neonate (newborn) or an infant or a baby.

What is sex?

Some people say sex is a trait, but sex is really a group of traits. A trait is a quality that a person can have. Examples of traits are eye color, height, or hair texture.

What we call “sex” is a simple way to talk about a group of traits. Many people think sex is a binary, which means “two.” Some people believe that calling sex a binary of male and female is too simple.

Sex is part of how a fetus develops. At this time, a fetus develops into someone who can make eggs, sperm, or neither. This is called sex development. These traits affect sex development:

  • genes: chemicals with a code that tells your body how to make the things your cells need. The code affects who you are (traits) and how you act (behaviors). Scientists estimate that genes account for between 25% and 50% of a person’s transgender traits. Learn more at genetics.
  • sex hormones: chemicals that can tell your cells how to help make gametes. They can also make your body ready to give your gamete or receive someone else’s gamete. Learn more at hormones. Sex hormones affect sex traits:
    • “Primary” sex traits
      • gonads: the organs in your body that can make gametes.
      • internal genitalia: organs inside your body that can help give your gamete or receive someone else’s.
      • external genitalia: outer organs that can help give your gamete or receive someone else’s.
    • “Secondary” sex traits
      • breast tissue
      • body hair including underarm and pubic hair
      • fat percentage and distribution
      • skin including scent and sweat
      • skeleton

For a long time, people would often look at the external genitalia of an infant right after the infant is born. Based on what they saw, they would often say “this baby is male” or “this baby is female.” Saying a baby is male or female is is called a sex assignment. Sometimes it is hard to tell just by looking at external genitalia. This is called a difference of sex development.

What is gender?

Gender means type or kind. Some languages have a gender for some words. The gender can be:

  • masculine
  • feminine
  • neuter

The idea of gender is also used for how types of people look and act. Some cultures think of gender as having only two types: masculine and feminine. This is called a gender binary. Some cultures have a third gender, so gender does not always match up with sex.

Some people who study the idea of gender say that gender is a social construction. They say that every culture makes up its own ideas about how gender is used on people. Some say that sex is a social construction, too. They believe that organizing people and culture on the basis of sex will change a lot this century.

Some people who study the idea of gender think of it like a rainbow, where all the colors blend. Imagine someone saying “There are only two colors: warm and cool.” Someone else might say, “There are seven colors: red, orange, yellow, green, blue, indigo, violet.” Someone else might say “There are 360 colors, one for each unit of visible light.” Someone else might say there are too many colors to count. People who think of gender as a rainbow call it a gender spectrum.

In a perfect world, everyone would be treated the same. Some people do not treat other people the same based on their sex assignment. They make up rules for how people should look and act based on their sex assignment. This is called a gender role. They also make up rules for how people should act toward others based on their gender. When these rules are not fair, it is called sexism.

For instance, these days many people test a fetus before it is born. One kind of test called an ultrasound can make an image of a fetus. That image can show the external genitalia. Before the infant is born, some people will choose how to treat the infant after it is born. They might buy clothes they consider “feminine” or toys they consider “masculine.” Those decisions are sexist.

What is gender identity?

When most children are around 3 years old, they start to form a gender identity. They start to think of themselves as part of a type based on gender roles in their culture. Many children form a gender identity that fits their sex assignment, but some do not.

What is gender expression?

Gender expression is how you look and act based on your culture’s gender roles. Most doctors who treat children say that letting all children look or act how they want is good for them. This is called an affirming model of care. They say it is better to support a child as part of healthy development.

When gender identity and sex assignment do not fit together

Some children form a gender identity that does not fit their sex assignment. This is called a difference of gender development. Most doctors who treat children say that these gender diverse children should not be punished or made to feel bad. Their gender identity and expression may change over time if they have support from parents.

Some people think our gender identity and expression can be a disorder or disease. Others think they are just how we feel and act (traits and behaviors). The two groups see the world in very different ways. They even use the same words in very different ways.

People who think gender is binary often think that gender identity is also binary. They often think gender diverse children are sick. They think the children can be cured by using shame and punishment. This is called a non-affirming model of care. They say that if this sickness is not treated, the child may become transgender. If it is treated, the child might not become transgender. People who think transgender people are sick call this cure a “desistance” or “detransition.” They often believe the affirming model is bad for children. The people who think that being gender diverse or transgender is a disease are not bad people. Even some transgender people think they have a disease. Good people can disagree.

Ethics are a set of rules about what is good and bad. Many people who work with children and transgender people think the affirmative model of care is good, or ethical. Many people who work with children and transgender people think the non-affirmative model is bad, or unethical. In some places, the non-affirmative model of care is against the law.

Why science matters

It is hard to talk about the science of gender identity and expression. Good science uses words and ideas that do not have bias. Bias means liking one idea more than others. Scientists should not have bias. Bias can cause bad science. Bad science can hurt people. That is why it is important to look for bias in science. This is even more true for science about people.

Some people say that transgender people hate science. This is a form of bias. Many famous transgender people are scientists. Many of us who are not scientists know a lot about how science works. We know that bad science has hurt transgender people in the past. We want all science to be good science.


There are a number of famous transgender biologists, including:

This site also covers cisgender biologists who have been involved in issues related to gender identity and expression, including:


Gender-Inclusive Biology (

  • Information for teachers and students

Learn more

The next page with easy words is transgender politics. This page is part of the information on academia.