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What Is “Sex”? What Is “Gender”? The difference between sex and gender

And How Can They Differ?

What follows is what we hope is a simplified but factual explainer of Gender, Sex and how some people are transgender. Science has evolved quite a bit since your parents went to school. And unfortunately, a lot of science curriculum concerning these things have been removed from schools. For deeper dives we have included a lot of links to scientific and medical journals and the peer-reviewed articles and studies contained. Things are not nearly as simple as Boys and Girls.

1. Biological Sex: A Set of Physical Traits

Biological sex refers to a collection of physical characteristics related to reproduction. These include:

• Chromosomes (XX, XY, and variations)
• Genes (such as the SRY gene)
• Gonads (ovaries or testes)
• Hormones (estrogen, testosterone, etc.)
• Internal reproductive organs
• External genitalia

Sex is usually assigned at birth based on visible genital anatomy. Most people are classified as female (typically XX chromosomes) or male (typically XY chromosomes). However, biological sex is not determined by one single factor — it is the result of multiple interacting systems
( www.ncbi.nlm.nih.gov/books/NBK279001/ ).

2. Chromosomes and DNA: How Sex Develops

Humans typically have 23 pairs of chromosomes. One pair are the sex chromosomes:

• XX → typically female development
• XY → typically male development

The Y chromosome contains a gene called SRY (Sex-determining Region Y). When SRY is present and active, it usually triggers development of testes during early fetal growth
( www.ncbi.nlm.nih.gov/books/NBK22246/ ).

If testes develop, they produce testosterone and other androgens. These hormones guide the formation of male internal structures and external genitalia. If SRY is absent (as in most XX embryos), ovaries develop and the fetus typically develops female reproductive anatomy
( www.ncbi.nlm.nih.gov/books/NBK279001/ ).

However, this pathway is more complex than “XX = female, XY = male.” Genes can vary. Hormone production or receptor response can vary. Chromosome patterns can vary.

Biologists recognize that sex is a multi-layered biological system, not a single binary switch
( www.pmc.ncbi.nlm.nih.gov/articles/PMC9355551/ ).

3. Intersex (Differences of Sex Development, DSD)

Some babies are born with variations in sex traits. These are medically referred to as Differences (or Disorders) of Sex Development (DSD) and commonly described as intersex variations.

Intersex can involve differences in:

• Chromosomes (e.g., XXY, XO)
• Gonadal development
• Hormone production
• Hormone sensitivity
• Genital anatomy

Examples include:

• Androgen Insensitivity Syndrome (AIS)

An individual may have XY chromosomes and testes but their cells cannot respond properly to testosterone. As a result, external genitalia may appear typically female despite XY chromosomes ( www.ncbi.nlm.nih.gov/books/NBK1429/ ).

• Congenital Adrenal Hyperplasia (CAH)

An XX fetus may produce unusually high levels of androgens, which can result in masculinized genitalia at birth ( www.ncbi.nlm.nih.gov/books/NBK278953/ ).

• Klinefelter Syndrome (XXY)

An individual with XXY chromosomes may develop typically male anatomy but with some variations in hormone levels and fertility ( www.ncbi.nlm.nih.gov/books/NBK482314/ ).

• Turner Syndrome (XO)

A person with a single X chromosome may develop typically female anatomy but with characteristic medical differences ( www.ncbi.nlm.nih.gov/books/NBK482322/ ).

These variations demonstrate that biological sex traits can develop in multiple combinations. Medical literature acknowledges that sex characteristics exist along a spectrum of biological possibilities
( www.pmc.ncbi.nlm.nih.gov/articles/PMC9355551/ ).

4. What Is Gender?

Gender is different from biological sex.

Major medical and psychological organizations define gender as a person’s internal sense of identity (man, woman, both, neither, etc.), as well as social roles and expectations associated with those identities

( www.apa.org/topics/lgbtq/gender-identity ).

The World Health Organization distinguishes:

• Sex → biological attributes
• Gender → socially constructed roles, behaviors, and identities

( www.who.int/health-topics/gender )

Gender identity develops through a combination of:

• Brain development
• Hormonal influences before birth
• Social environment
• Psychological development

Research in neurobiology suggests prenatal hormones influence brain structures related to gender identity ( www.pmc.ncbi.nlm.nih.gov/articles/PMC6677266/ ). However, gender identity is not reducible to chromosomes alone.

5. How Can Someone Born With Male Genitals Be Female?

A person assigned male at birth (based on genital appearance) may later identify as female. This is known as being transgender.

Key points:

1. Sex assignment at birth is based on visible anatomy.
2. Gender identity refers to internal identity.
3. These do not always align.

Medical research shows that gender identity appears to have biological components influenced by prenatal brain development and hormones ( www.pmc.ncbi.nlm.nih.gov/articles/PMC6677266/ ).

Importantly, gender identity is recognized by major medical organizations as a deeply rooted aspect of human identity, not a voluntary preference ( www.apa.org/topics/lgbtq/transgender ).

This means a person can:

• Have XY chromosomes
• Have male genitalia at birth
• Develop a female gender identity

Or vice versa.

Biology is complex enough that chromosomal sex, hormone exposure, and brain differentiation do not always align perfectly.

6. What Do DNA and Chromosomes “Say”?

Chromosomes provide part of the biological blueprint — but they do not operate alone.

Sex development involves:

• Chromosomal signals (XX, XY, variants)
• Gene activation (SRY and others)
• Hormone production
• Hormone receptor function
• Tissue response to hormones

Mutations or variations in any step can produce different outcomes ( www.pmc.ncbi.nlm.nih.gov/articles/PMC4077654/ ).

For example:

• An XY individual without a functioning SRY gene may develop ovaries.
• An XX individual exposed to high prenatal androgens may develop masculinized anatomy.
• An XY individual with complete androgen insensitivity may develop female-typical external anatomy.

Therefore, chromosomes are important — but they are not the sole determinant of sex traits.

Biologists describe sex determination as a developmental process, not a fixed binary switch

( www.pmc.ncbi.nlm.nih.gov/articles/PMC9355551/ ).

7. Key Clarifications

• Most humans fall into typical male or female categories.
• A measurable minority are born with intersex variations.
• Biological sex is based on multiple interacting systems.
• Gender identity refers to a person’s internal identity and social experience.
• Sex and gender usually align — but not always.

Major medical bodies recognize both intersex variations and transgender identities as natural aspects of human diversity ( www.wpath.org/publications/soc ).

8. Summary

Biological sex involves chromosomes, genes, hormones, anatomy, and development. While most people are born clearly male or female, variations exist because development is biologically complex.

Gender refers to identity and social roles. It is influenced by biology but is not defined solely by chromosomes or genitals.

Because sex traits and brain development involve multiple steps, it is biologically possible for:

• A person with male genitalia to identify as female.
• A person with female genitalia to identify as male.
• A baby to be born with intersex traits due to chromosomal, hormonal, or genetic variations.

Modern medicine and biology recognize that sex is more complex than a simple binary classification and that gender identity is a real and deeply rooted aspect of human identity.

Top 10 Science-Forward Links

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC10842549/ — Sex complexity beyond binary definitions
2. https://my.clevelandclinic.org/health/articles/16324-intersex — Intersex biological variation explained
3. https://www.ncbi.nlm.nih.gov/books/NBK279001/ — Sex differentiation and hormones in development
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC4077654/ — Evolutionary biology of sex determination
5. https://www.ncbi.nlm.nih.gov/books/NBK22246/ — SRY gene basics (Y chromosome)
6. https://www.sciencedirect.com/science/article/pii/S0303720725000097 — SRY variants & DSD cases
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC9355551/ — Sex & gender beyond the binary
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC11335805/ — Sex vs gender biological/social distinction
9. https://en.wikipedia.org/wiki/Intersex — Intersex overview & definitions
10. https://www.verywellhealth.com/sex-vs-gender-meaning-5093122 — Clear distinction sex vs gender (lay)

1️⃣ Definitions (Plain Language)

Biological Sex

• Refers to physical traits, including chromosomes, hormones, and internal and external anatomy.
• Most people are classified as female (typically XX) or male (typically XY) at birth.

Gender

• Refers to one’s internal sense of self — e.g., woman, man, nonbinary — and is influenced by culture, development, and personal identity, not solely biology.

Intersex / DSD (Differences of Sex Development)

• Natural biological variations where sex characteristics don’t fit typical binary definitions (chromosomes, hormones, anatomy).

2️⃣ How Sex Develops Biologically

Timeline of Development

• Fertilization: Chromosomes (XX or XY) set genetic sex.
• Gene Expression: The SRY gene on Y triggers testis development — leading to male pathway.
• Hormone Influence: Testes produce testosterone → external genital differentiation. Without it → female development.

3️⃣ What Determines Gender?

Brain & Identity

• Gender identity is a psychological and social experience, shaped by a complex mix of prenatal biology, brain development, culture, and personal sense of self.

Important Distinction
Sex ≠ Gender — biological traits are not the same as how a person identifies.

Academic Bibliography (Peer-Reviewed DOIs + Summaries)

1. Rehmann-Sutter C. Is sex still binary?
   → Argues that biological sex can be more complex than binary categories. (PMC10842549)
2. Bachtrog D. et al. Sex Determination mechanisms across species.
   → Reviews how diverse sex determination systems evolved across organisms. (PMC4077654)
3. King DE. et al. Inclusion of Sex and Gender Beyond the Binary.
   → Shows biological variation exists and that sex may best be understood as multi-trait. (PMC9355551)
4. Quintana GR. et al. Sex vs Gender separate identities.
   → Distinguishes biological sex as genetic/anatomical and gender as sociocultural identity. (PMC11335805)
5. Roselli CE. et al. Neurobiology of gender identity & sexual orientation.
   → Reviews prenatal hormones/genes’ influence on brain and identity. (PMC6677266)
6. StatPearls / Aatsha PA. Embryology sexual development & SRY.
   → Details the role of SRY and early development of testes/ovaries. (NCBI Bookshelf NBK557601 — not publicly DOI but research-linked)
7. Sex chromosome evolution studies.
   → Sex chromosomes evolve and do not always define fixed binary outcomes. (PMC3509754)
8. Intersex genetic condition reviews.
   → Mutations in sex determination genes cause DSD; biology is medically diverse. (Turn0search6 and relevant clinical literature)

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About Post Author

Chrome Tuna

bobscutler@gmail.com

https://chrometuna.com

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