It seems like a pretty clear system – but it wouldn’t be biology without exceptions and extra rules muddying the waters.
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Thus, the presence of a single Y chromosome switches on the male pathway, something that is clear in what’s called Klinefelter Syndrome, in which individuals carry two X chromosomes and one Y chromosome, but develop testes and appear generally “male.” Without the presence of a Y chromosome, and thus without SRY, cells secrete estrogen instead of testosterone, and an XX baby develops female sexual organs. In this way, SRY is like the master switch to turn on the suite of “male” genes in a developing organism. SRY is a transcription factor – a genetic element that can turn on the expression of other genes. The “default” sex (i.e., without any other further input) is actually female – however, the presence of a gene called SRY on the Y chromosome initiates the release of testosterone and the formation of male sex organs. At first, its sexual organs manifest as a genderless gonad, or sex gland – basically a small, thick ridge of tissue near what will become the abdomen. įollowing fertilization, a fetus begins to develop. Thus, the contribution from the father determines the sex of the baby. When egg meets sperm, each parent contributes 22 non-sex chromosomes and one sex chromosome – always an X from the mother, and either an X or Y from the father. Briefly, human cells all carry chromosomes, which carry our genes. The XY sex determination system ( Figure 1A) is certainly what’s most familiar to us, and it’s used in most other mammals, as well as a few select insects and plants. We were all taught the classic recipe in grade school: an X chromosome from mom and an X chromosome from dad will yield a genetic female, while an X chromosome from mom and a Y chromosome from dad will yield a genetic male. What are the ways in which sexual characteristics are encoded? Why are there so many systems for one seemingly common result? SRY not SRY In most sexually reproducing organisms, there are two sexes – but the ways in which these sexes are determined and the ways in which they manifest vary greatly.
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Compared to asexual reproduction, in which cells can simply create carbon copies of themselves, sexual reproduction allows for the introduction of genetic diversity into a population.
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Most multicellular organisms, humans included, use sexual reproduction to reproduce. The sex determination we’ll discuss today is (unfortunately?) not the dogged resolve to copulate. With regards to science, we don’t know enough about gender identity to draw any conclusions about its biological underpinnings, and certainly not about what is “right” or “wrong.” We are only now beginning to fully understand how mammalian sexual identity has evolved, and its dependence on the sex determination systems that allow biological development of sexual characteristics in different organisms. It’s a complicated subject, to say the least. Sexual identity has been in the news often lately, and unsurprisingly so: the past few years have yielded sweeping reforms in civil rights, spurring new conflicts surrounding everything from age-old battles in gender equality to legislation enforcing anti-transgender bathrooms.
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Not intercourse, though – more about how genetic sex is programmed during development.