8.6 Human Sex Development

When a sperm and egg join to form a zygote, whether that sperm carried an X chromosome or a Y chromosome determines the eventual sex of the developing fetus.  However, for the first seven weeks of development a male and female fetus are indistinguishable.

Shortly after the seventh week, the presence of a gene called “Sry” on the Y chromosome directs the development of the testes. These new testes then begin to produce testosterone (a hormone associated with male development) and another hormone called Müllerian inhibiting hormone; together, these two hormones cause the disintegration of the female internal organs and further the development of male internal and external anatomy.  In the absence of the Sry gene and the Müllerian inhibiting hormones, the female external and internal anatomy will form.

Figure 8.20 External genital development of a human fetus. Note the undifferentiated genitals at 7 weeks, and the increasing differentiation as development continues.


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This interactive website will walk you through the steps of sex differentiation in males and females

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  1. https://www.ncbi.nlm.nih.gov/books/NBK10943/bin/ch30fba1.jpg


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The Evolution and Biology of Sex Copyright © 2020 by Sehoya Cotner and Deena Wassenberg is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.