Written by Eric Olson
2 min read
Are Human Embryos All Female to Begin With?
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Try for freeThe idea that all embryos begin as female before some transform into males sounds fascinating—but is it scientifically accurate? We asked a top reproductive biology expert to unravel this developmental mystery, and the answer challenges everything many of us thought we knew about our biological beginnings.
Taylor Pini has answered Extremely Unlikely
An expert from University of Sydney in Reproductive Biology
No, the sex of the embryo is determined at fertilization and depends on whether the sperm that fertilizes the egg carries either an X or a Y chromosome (sperm only carry one or the other because they are “haploid” cells). All eggs carry an X, because females are XX. If the fertilizing sperm carries an X, the resulting embryo will be XX (female) and if the fertilizing sperm carries a Y, the resulting embryo will be male (XY).
What this video talks about is the activation or “switching on” of a special gene (called SRY), carried only on the Y chromosome. XX and XY embryos essentially undergo the same development up until the SRY gene is turned on. Once this gene is activated, development of XY embryos is steered towards producing testes rather than ovaries (i.e. male reproductive organs), and will not produce a uterus or fallopian tubes (i.e. female reproductive organs).
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