Men Are Men, Women Are Women: It's Not So Simple

The thing that makes a human male a male and a female a female is not as clear-cut as you might think. I recently ran across this article on the complexity of XY, XX, XXY, XYY and more:

...this paper [looked] at the SRY genes that are shared by a father and his daughter. In this case the daughter has XY "male" chromosomes, but the SRY genes didn't trigger the switch in the predictable way so instead she developed internal female genitalia (ovaries, fallopian tubes, and uterus).

The researchers assumed that something significant must happen to make the SRY switch function in this unexpected way (in their language they expected that a "severe insult to the Y-encoded switch" was necessary and would be in the neighborhood of a factor of 100 or more). What they found was the threshold at which the SRY functions in this unexpected way was only a factor of two. From the release:

"Therefore, human males actually develop near the edge of sexual ambiguity. This means that, unlike the robust genetic programs which develop other essential processes like heart function, the SRY gene master switch is particularly vulnerable to change. It only takes a slight deviation from the normal process to dramatically alter fetal sexual development."

Given the importance of sexual reproduction to the survival of a species, why do human SRY genes function so close to the boundary of infertility? The idea of an unreliable master switch might appear paradoxical, but a growing body of research suggests that it might be an evolutionary necessity.

Weiss hypothesizes that, rather than predictability, diversity in sex development and expression is itself an evolutionary advantage:

"We have this tenuous switch on the Y chromosome, and we anticipate that its gift to humanity is variability in the pathway of male development from its earliest stages. The essential idea is that our evolution has favored a broad range of social competencies. In prehistory, this range would have given a survival advantage to communities enriched by a diversity of gender styles."

Very interesting.