There are at least 4 different ways that scientists define biological sex, none of which create a sex binary.
Of course, the presence of intersex people makes this clear without further analysis: Female <–> Intersex <–> Male.
The methods used by scientists to define biological sex include:
1. Chromosomal sex – This refers to a person’s chromosomal complement. While XX and XY are the most common, there are a diverse range of naturally occurring variations, including XXY, XXXY, XYYY, and others.
Genes typically found on the Y chromosome (such as the SRY gene) can also cross-over during development to the X chromosome, produce “XX males”.
Additionally, sex chromosome mosaicism (mixed genetic cells) is possible.
2. Gonadal sex – This refers to gonadal presentation, which can be ovaries, testes, both, or neither. Combinations of both can be present, either as a single structure (an ovotestis), as separate structures, or as mixed cell types.
Gonads produce gametes (egg/sperm), which also cannot be used to create a sex binary in humans because no gametes can be present along with ambiguous or mixed gonadal tissue.
3. Phenotypic sex – This refers to observable characteristics, which include body size and shape, as well as the secondary sex characteristics such as body hair, facial hair, and voice.
On a daily basis, most non-scientists (society at large) assume other people’s biological sex based on phenotypic traits.
4. Behavioral sex – Behavioral sex refers to the way an individual behaves within their species.
Regardless of the method used, biological sex is determined by a constellation of traits, none of which are binary.
These traits include but are not limited to: external genitals, internal genitals, gonads, chromosomes, sex hormones, androgen receptor sensitivity, gene expression, skeletal structure, brain structure, and secondary characteristics.
Finally, biological sex and gender diversity has been documented across human history and is present in most species worldwide.
*This article is part of a Biological Sex and Gender Diversity Series published by Cade Hildreth, B.A., M.S. You can read the other articles here:
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Gender Spectrum: A Scientist Explains Why Gender (and Sex) Aren’t Binary
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Does the Y Chromosome Create a Sex Binary? Science Says No
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Biology Rejects a Sex Binary: Gametes (Egg/Sperm) Aren’t Binary, After All
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Sex Isn’t Binary: International Olympic Committee (IOC) Confirms Sex to Be a Spectrum