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[–]MarkTwainiac 6 insightful - 1 fun6 insightful - 0 fun7 insightful - 1 fun -  (0 children)

[Note: I made extensive additions to this post on Jan 16 to expand on the points made originally.]

To add to my earlier comments, I want to make it clear that there's a lot more to determining the sex of a plant or animal than just developing the anatomy early in life that will give an individual organism the potential capacity to generate/mature/release either sperm or ova at some later point in life. The gamete definition is the one biologists use coz it's the simplest and it applies universally to all sexually-reproducing species, plants and animals alike. But it's really only the starting point.

In utero, human embryos/fetuses whose development is typical will develop a lot of additional reproductive anatomy beyond the gonads from whence gametes eventually will issue.

In addition to beginning to develop testes in the 7th week, a human embryo on the male pathway that becomes a fetus typically will develop ancillary sex organs that include a penis and a prostate. The reproductive purpose of these ancillary sex organs is to facilitate the ejection of sperm from a male body and delivery of it into a female body so that the sperm will have a chance of meeting up with and possibly fertilizing an egg. For example, sperm can only pass through the male urethra and out the tip of the penis with the aid of fluids made by the prostate and other male glands.

In addition to developing ovaries in the 7th week, a human embryo on the female pathway that becomes a fetus typically will develop additional sex organs such as Fallopian tubes, a uterus (which includes the cervix), a vagina and a vulva (which itself has different parts, such as labia and clitoris). The reproductive purpose of these other sex organs is, first, to facilitate the entry of sperm into the female reproductive tract for the purpose of sperm reaching the Fallopian tubes and fertilizing an egg there. If fertilization occurs, the egg will travel to the uterus and become implanted in the uterine wall, then if the uterus grows a placenta, the fertilized egg will have a chance to develop into an embryo, then a fetus, and finally to become a baby. Once a human fetus in utero becomes old enough to survive outside the mother's body, labor will begin as the uterus contracts and the cervix - the neck of the uterus leading into the vagina - dilates many times larger than its normal size until it's about 10 cm wide, equivalent to the size of a standard coffee can lid or a bagel. Customarily, a mother will give birth via her vagina functioning as the "birth canal."

If if in any ovulatory cycle, the egg matured and released by the ovaries is not fertilized and not successfully implanted in the uterus, or no egg is released at all, the lining of a female human's uterus will slough off and pass out of the body in a process called menstruation. Allowing menses to exit the body is another of the reproductive functions of the very versatile human vagina.

But sex of course is not just about our gonads and other reproductive anatomy. The general rule is that human beings are determined to be male or female based on a list of five factors that are checked at or shortly after birth, and/or during utero, and often later in life as well, via a number of different means.

The five factors on he checklist are:

  • sex chromosomes, usually meaning XX or XY, but more specifically the presence or absence of a Y chromosome, more specifically the SRY gene normally on the Y;

  • the type of gonads (testes or ovaries);

  • the anatomy of the external genitals including arrangement of urinary anatomy (vulva and female urethra vs testicles, penis and male urethra );

  • the internal reproductive organs a person has (such as vagina, uterus, Fallopian tubes in females and prostate in males);

  • the individual's sex hormones and sex hormone receptors.

The means for checking for these factors have always included physical examinations of newborns that involve looking carefully at their genitalia, and examinations and other investigations later on as well if during adolescence or adulthood a person's secondary sex characteristics and bodily processes depart from the norm (for example if a girl doesn't start menstruating by her mid-teens, or if a boy's testicles and penis don't get larger during puberty.) But nowadays, it's also very common for additional means to be used to ascertain human sex as well. The additional means now in wide use include genetic testing of fetuses in utero and of infants following birth; scanning of fetuses whilst still inside their mothers' bodies (pregnant women today commonly get fetal sonograms by 20 weeks, often earlier); scanning of individuals' bodies later in life when outside their mothers' wombs to ascertain exactly which internal organs they have; endocrinological testing to test for the presence and levels of an array of sex hormones. Also, sex anatomy is confirmed by standard medical procedures done for other purposes over the course of life, such as the standard internal pelvic exams girls and women get when being fitted for certain birth control devices or tested for cervical cancers (Pap smear) and the prostate checks older men routinely get after age 50 or so.

It used to be thought that human sex differentiation began in utero began when the gonads first start developing at around 7 weeks, and that all the other sex differences were primarily the result of the sex hormones that issued from the developing gonads. But it's now been found that sex differences can be observed in the primitive progenitor cells of embryos much earlier than 7 weeks - they've been found at 6-7 days so far, and with more research it will probably turn out that they exist even earlier. Since sex differences once thought to follow from and be the result of gonadal differentiation and gonadal sex hormones have been shown to actually predate both, the theory is that many physical sex differences are directly caused by the sex chromosomes themselves.

Not only do sex chromosomes exist in every single cell in our bodies, it's become abundantly clear in recent years that they affect not just how our sex organs develop and work, but how ALL our organs and body systems develop and function. The diseases and medical conditions we are susceptible to, the trajectory of those conditions and diseases, the treatments and the likely outcomes, are all strongly influenced by our sex chromosomes.

For example, whilst COVID-19 affects both sexes, and in many countries more women than men have gotten it, the likelihood of needing ICU care and dying from COVID is much higher in persons with XY chromosomes than persons with XX chromosomes even when all other factors (age, obesity, underlying health conditions, race, etc) are the same. I haven't checked recently, but when I looked at the issue in the late summer and early fall, males were twice as likely to die of COVID than females, and three times as likely to end up in an ICU due to it. The theory is that this is due to the fact that the genes that confer and regulate human immunity (as well as that do many other things) are all on the X chromosome, and males only have one X chromosome whereas females have two, males have lowered immune function compared to females. Which brings us to another interesting fact: male scientists have always assumed that in female humans, one of the X chromosomes automatically gets deactivated or "goes silent" at or shortly after conception. But in recent years, this has been proven not to be true. At all. In fact, in many of our organs and body systems, the second X chromosomes in female humans remains active throughout life - and in others where one X is normally "silent" or inactive, the second X can become active when need be, functioning as a backup if the first X chromosome isn't doing its job properly.

BTW, the entirely wrongheaded assumption that male scientists and physicians have made for generations about how XX chromosomes work in female humans just goes to show how much sexist bias against females there has always been in science and medicine - and that many of the "truths" scientists and doctors long have spouted about how female bodies function are a bunch of baloney.