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My friends and I were bored enough to try out this activity.

Picture a teenage boy in your mind.

From this picture, remove any accessories (glasses, piercings, etc.), moustache or beard, then replace the hairstyle with that of a stereotypical teenage girl.

Are you possibly convinced if someone tells you that, this picture was originally of a teenage girl?

I tried it by picturing a few boys who are my friends, but still, I was never convinced because I feel that there's something inherent about the facial features of these boys that made them appear boyish rather than girlish.

If this is not some psychological effect caused by my acquaintance with the boys I pictured (as I know them well so I cannot accept the mapping of their faces to the opposite gender), then are there really biological factors, genes in sex chromosomes in particular, that result in differences in the development of facial features between males and females?

Apparently, "facial hair" isn't an acceptable answer...

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Yes, having a Y chromosome does cause specific alterations to facial structure.

The Y chromosome doesn't contain much actual genetic information. Most of the information needed to activate the male developmental program (the female one is the default) is in just one gene 'SRT', the other genes on the Y chromosome aren't super important, such that you can get a normal looking (but infertile) man if this gene is transferred onto an X chromosome.

This SRT acts as a switch causing various changes, but one of the most important ones is the higher production of testosterone. Testosterone (and some other hormones called 'androgens' cause changes to a lot of things, and one of them is facial morphology. In fact weirdly, the bones in the face retain the capacity to grow in adults, so you can give someone hormone treatment and their facial structure will change (google this and you'll clearly see the difference in people's facial shape after hormone therapy for sex change procedures - especially around the jaw). There is also some correlation, in both sexes, between the amount of testosterone in someones blood and the 'manliness' of their face.

Your teenage friends are currently undergoing a natural process similar to artificial hormone treatment, and their faces will get more and more male looking as puberty goes on.

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  • $\begingroup$ Any reference ? $\endgroup$
    – Ebbinghaus
    Commented May 3, 2016 at 16:32
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    $\begingroup$ For which claim? Most of that is pretty basic. For the association with T and male facial characteristics see e.g. this $\endgroup$
    – Dermot Harnett
    Commented May 3, 2016 at 16:41
  • $\begingroup$ @DermotHarnett - +1 for this answer. Usually when someone requests a source, it's good to edit it into your answer (per site guidelines). There are lots of papers to support this answer even more strongly, and even starting with testosterone exposure in utero (e.g. Whitehouse et al, Prenatal testosterone exposure is related to sexually dimorphic facial morphology in adulthood). $\endgroup$
    – anongoodnurse
    Commented May 3, 2016 at 16:56
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    $\begingroup$ @DermotHarnett I'd be wary of claiming the "default" is female, considering (among other things) a single X chromosome will result in Turner's syndrome, not a characteristically distinct female or male. $\endgroup$
    – Harris
    Commented May 3, 2016 at 18:42
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The answer is a bit of a 'yes and no', though mostly 'no.

The Y chromosome has very few genes, but arguably the most pivotal is the sex-determining region Y or SRY gene, also known as TDF or testis determining factor. The protein this gene codes for acts essentially as a master switch for male phenotypes by triggering the development of testes in the developing fetus.

One of the primary function of the testes themselves (aside from sperm production), is the production and release of testosterone. It is the testosterone itself that causes masculinization/male phenotypes. Among these masculine phenotypic changes are a modified subcutaneous fat distribution and a thickening and widening of the jaw bone.

So in this respect, yes, the Y chromosome contains genes that influence facial structure. However, the influence really occurs at a regulatory level, and there are no 'facial structure genes' per sé on that chromosome. In fact, having a deleted on nonfunctional SRY gene, among other potential causes), leads to a person having an XY karyotype, but often being phenotypically female (a lack of a second X chromosome in this case, however, usually leads to other functional problems including infertility).

Further, there is also a condition known as Androgen Insensitivity Syndrome, wherein a person's cells lack functional receptors for testosterone. It is possible in this case, for a person to have an otherwise fully functional Y chromosome (including a working SRY gene), but be otherwise biologically female, as it is only the androgen hormones, to which the person's body does not react, that lead to any masculinization in the first place.

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