Do We All Start Out As Females?

Did you know that we all started out as female embryos? Well, not exactly, but it's a common misconception that has some truth to it. Let me explain.

The sex of humans is determined from fertilization, that is, as soon as it becomes a zygote: if it is XX, it will be a female, while, if it is XY, a male. However, in the beginning, human embryos have the precursors of both female and male gonads, called the Wolffian duct and the müllerian duct, respectively.

These ducts are responsible for developing into the reproductive organs, such as the ovaries or testes, and the internal genitalia, such as the uterus or prostate. But they don't do that right away. They wait for a signal from the genes.

One of these genes is called the SRY gene, which is located on the Y chromosome. This gene activates around the fifth or sixth week of embryonic development and triggers the production of testosterone and anti-müllerian hormone (AMH). These hormones cause the Wolffian duct to grow into the male reproductive system and the müllerian duct to regress and disappear.

If the SRY gene is not activated, either because there is no Y chromosome or because of a mutation, the opposite happens: the Wolffian duct regresses and the müllerian duct develops into the female reproductive system. This means that without the SRY gene, the embryo will follow the female pathway by default. So in a sense, nature prefers females over males.  This makes sense because females are the ones who do most of the reproduction.  

But what about the external genitalia, such as the penis or clitoris? These also depend on hormones, but they develop later, around the ninth week of embryonic development. Testosterone causes the genital tubercle to elongate into a penis and the labioscrotal folds to fuse into a scrotum. Without testosterone, the genital tubercle becomes a clitoris and the labioscrotal folds remain separated as labia. So we all start with the potential of being either male or female with nature preferring female by default. This is why some say that we are all females at the onset of life. To be male, the Wolffian system has to be activated, however, to be a female nothing needs to be activated. The embryo will become a complete female as it gestates. 

And what about nipples? Why do males have them if they don't need them? Well, nipples form before the SRY gene kicks in, during those fateful five or six weeks when we all look more or less the same. Nipples are part of the basic body plan that we inherit from our mammalian ancestors. Only females end up with breasts attached to them because of another hormone called estrogen.

So, to summarize, we all start out with both male and female potential in our embryos, but our genes and hormones decide which one will prevail. The SRY gene on the Y chromosome is the main switch that determines whether we will develop as males or females. Without it, we would all be females by default.

I hope you enjoyed this blog post and learned something new. If you want to know more about this topic, you can check out these references:

What do you think? Post below on Disqus and be sure to follow the rules so your comment can be allowed.

References:

: [WATCH: We Were Once All Female : ScienceAlert](https://www.sciencealert.com/watch-we-were-once-all-female)

: [Embryos aren't female by 'default' after all, study shows - Genetic Literacy Project](https://geneticliteracyproject.org/2017/09/21/embryos-arent-female-default-study-shows/)

: [Do all human fetuses begin as female? - inviTRA](https://www.invitra.com/en/faqs/do-all-human-fetuses-begin-as-female/)

: [All Mammals Start as Female - Fact or Myth?](https://factmyth.com/factoids/all-mammals-start-as-female/)

: [When Does a Baby Develop Gender? | Hello Motherhood](https://www.hellomotherhood.com/article/231357-when-does-a-baby-develop-gender/)