Sociology

You Can’t Always Tell Someone’s Chromosomes by Looking at Them at Birth

Doctors cannot always identify biological sex by appearance at birth. Chromosomes, genes, and hormones interact in complex ways. This leaves many people discovering their true label much later in life.

T Jackson

28 Jan 2026 — 4 min read

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This topic may get me a few unsubscribes, but it is irresponsible not to acknowledge the facts.

Abstract

Most people, lacking knowledge, assume that a baby’s biological sex can be identified at birth simply by examining their body. Often, this is true. Doctors almost always label a baby at birth as male or female based on external features. Yet, human biology is more complex than it seems. A person’s chromosomes, hormones, and internal organs do not always match what is visible on the outside. As a result, determining whether an individual has male (XY) or female (XX) chromosomes solely on the basis of appearance is often impossible.

Chromosomes are structures within cells that carry genetic information, also known as the code of life. It is commonly taught that females have the XX chromosome configuration, whereas most males have the XY chromosome configuration. These chromosomes usually determine whether the body develops ovaries or testes, along with typical female or male anatomy. However, differences in genes or hormones can alter how the body develops.

Early in pregnancy, all embryos have the potential to develop either male or female reproductive organs. A gene called SRY, which is usually found on the Y chromosome, starts the development of testes and male hormones. If this gene functions as intended, the body follows the male pathway. If it does not work, or if the body cannot respond to male hormones, development may follow the female pathway.

Yes, people with XY chromosomes can naturally develop bodies that appear female. They may be born with a vagina and other female features, despite typically having male chromosomes. In some cases, they may also have a working uterus. On the other hand, some individuals with XX chromosomes develop male-appearing bodies when the SRY gene is present on one of their X chromosomes. They may be born with a developed penis and testes even though they do not have a Y chromosome.

These conditions are examples of what doctors call differences in sex development (DSDs). DSDs refer to a group of medical conditions where the development of chromosomal, gonadal, or anatomical sex is atypical. These conditions are rare but highlight that determining sex is not always straightforward or based only on appearance.

In many cases, babies with these conditions look completely typical at birth, with doctors not noticing anything physically unusual. The person grows up believing they are biologically male or female, as others do. It is often not until puberty or adulthood that questions arise. Some may not start menstruation, even though they look female. Others may not develop typical male traits. A chromosome test at birth, before assignment, is the correct scientific method for labeling.

For many people with this condition in the USA, the diagnosis arises only after repeated attempts to have children. They learn they are infertile due to an inability to produce eggs or sperm. Medical testing may reveal unexpected chromosome patterns. This can be highly emotional, especially for individuals who remain unaware until adulthood. Assisted reproduction options such as IVF, donor eggs, or surrogacy can still provide assistance for some individuals to have children.

This raises questions about whether famous historical figures like Queen Elizabeth I, King William III, Emperor Joseph II, and Charles II had differences in sex development that went unrecognized in their time. It’s interesting to think about whether US presidents like James Buchanan, George Washington, James Polk, and Andrew Jackson had undiagnosed conditions. This is likely due to the limited scientific knowledge of their time.

Visual observation alone is not enough to identify a person's biological sex. The external appearance does not always reflect genetic or hormonal makeup, as chromosomes, genes, and hormones interact in complex ways to shape development from embryo to adult.

Scientists estimate that visible differences between a person’s physical sex traits and their chromosome pattern occur in less than 1% of the population. Variations in sex development affect about 1–2% of people. Most cases aren't noticeable at birth. Many adults find out about these differences later through medical testing. This reinforces the point that external appearance does not always reflect underlying biology. Using the 1% factor means a metropolitan area the size of Dallas-Fort Worth can have as many as 8,300 people with DSDs.

Understanding this complexity is important for both medical care and social awareness. It reminds us that human biology exists on a spectrum rather than in strict binary categories. While most people fit typical male or female patterns, some do not; this is a natural part of human variation. Reducing it to a strict binary view is not evidence-based. Therefore, sex (male or female) should not be listed on a birth certificate unless a chromosomal test is performed and the results match the appearance.

In conclusion, a person’s chromosomal set cannot always be determined by examining them at birth. Some people with XY chromosomes may develop female bodies, and some with XX chromosomes may develop male bodies. Many live their lives without knowing until they face fertility problems later on. This shows that sex development is more complex than visual labels. Science is still exploring the true mechanisms of our body's development.

References:

https://isna.org
https://www.adnpfoundation.org/understanding-genetics.html
https://wi.mit.edu/news/day-life-graduate-student-and-genomics-researcher-neha-bokil
https://plato.stanford.edu/ENTRIES/feminist-philosophy-biology/
https://pubmed.ncbi.nlm.nih.gov/1516467/
https://medlineplus.gov/genetics/gene/sry/
https://www.childrenshospital.org/conditions-treatments/androgen-insensitivity
https://my.clevelandclinic.org/health/diseases/disorders-of-sexual-development
https://academic.oup.com/jes/article/9/8/bvaf099/8169438