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The science of transgenders

Q: Can one identical twin be a transgender while the other one is normal?

Krishna:

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Let us first understand the concept of transgender.

Transgender people are not "formed"; rather, a person is transgender when their internal gender identity does not match the sex they were assigned at birth.

Research suggests that gender identity has strong biological roots that develop before birth.

Science supports the idea that being transgender is rooted in biology—specifically a mix of prenatal hormone exposure and genetics—and not necessarily caused by a child's upbringing. However, separating womb hormones entirely from genetics is inaccurate, as the two work closely together. (1)

Role of hormones:

Physical sex characteristics develop early in pregnancy, whereas the sexual differentiation of the brain happens later.

Variations in how the body processes hormones like estrogen and testosterone during early development in the womb can affect how the brain's neural pathways develop regarding gender identity

Scientific models suggest that variations in prenatal hormone levels (like testosterone) or how the fetal brain's receptors process those hormones can lead to a brain identity that does not match the physical body. (2)

Studies on individuals with prenatal endocrine conditions, such as congenital adrenal hyperplasia (CAH), show higher rates of gender variance, supporting the impact of intrauterine hormone environments.(2)

Role of genetics:

Current research points to a combination of genetic and multi-factorial biological influences rather than a single environmental cause.

Recent twin studies demonstrate that identical twins (who share 100% of their DNA) show higher concordance rates for being transgender than non-identical twins or siblings, proving that genetics have a real foundational influence. (3)

Research has identified specific genetic variants in sex-hormone-signaling pathways (which process estrogen and testosterone) that appear more frequently in transgender individuals (4)

Transgender vs. Intersex

It is common to confuse being transgender with being intersex, but they are entirely different concepts.

Transgender describes a mismatch between a person's physical body/assigned sex and their internal sense of identity.

Intersex refers to being born with reproductive or sexual anatomy that does not fit the typical definitions of female or male. Most intersex people identify with the sex they were assigned at birth, though some may later identify as transgender.

So, yes, one identical twin can be transgender while the other one is normal.

Why this happens

Distinct Brain Development: Gender identity forms in the brain through exposure to prenatal hormones, which can affect each twin differently in the womb even with shared DNA.

Genetic Expression: While identical twins share nearly all of their genetic code, environmental factors, epigenetic differences and minor mutations change how those genes are expressed.

Individual Identity: Twin studies show that if one identical twin is transgender, the other is only about 21% to 33% likely to be transgender as well, meaning they frequently have different gender identities. (3)

There are several real world examples of both identical twins and non-identical twins differing in their identifications with their actual genders.

And one identical twin can be intersex while the other appears typical, though it is exceedingly rare.

How this Happens

Identical (monozygotic) twins start from a single fertilized egg. However, genetic and developmental variations can occur after the egg splits.

Post-zygotic mutations or chromosome loss: If an embryo with male (XY) chromosomes splits, one twin might lose a sex chromosome (becoming XO, associated with Turner syndrome) or experience uneven chromosomal distribution during early cell division (mosaicism).

Chimerism and Sesquizygotic development: Rare documented medical cases—such as a famous study published in Human Genetics via PubMed—describe sesquizygotic or chimeric twins sharing maternal DNA where one twin developed as an intersex individual with ambiguous genitalia (true hermaphroditism) and the other developed as a typical male.

X-linked or epigenetic differences: Variations in how identical genes are expressed (epigenetics) or secondary genetic conditions can affect physical sex development in only one of the twins.

Footnotes:

1. https://www.biorxiv.org/content/10.1101/2023.02.27.530343v2.full

2. https://pmc.ncbi.nlm.nih.gov/articles/PMC6677266/

3. https://theconversation.com/critics-claim-trans-people-defy-biology...

4. https://hudson.org.au/news/written-in-dna-study-reveals-potential-b...

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