The Relationship Between X and Y

January 26, 2010

X and Y have a relationship – the nature of which often varies (depending upon who and when you ask – and what (variables) X and Y represent. Other letters of the alphabet are used in mathematics, genetics, and many other (human) sciences, but X and Y are the most commonly used when it comes to expressions of relationships (of any kind). You’ve probably heard women talk abut their “ex” (boyfriend/husband/partner) and men talk about their “why” (for almost everything except their own blame – especially after relationships end).

He said, She said: Male and Female thinking and communication is often quite different – especially when it relates to the “other” sex. Differences between men and women are mainly the result of the X chromosome.

In nature, animals produce different numbers of male and female offspring depending on their social status. In human society, female members of social “elites” often pair off with men considerably older than themselves – and have more sons than daughters.

Biblical accounts of the order of creation aside, the default sex of (everything in) life is female! Humans have 23 pairs of chromosomes – that determine everything about the individual. Half of each chromosome pair is inherited from the mother, and half from the father. One of the pairs determines a person’s sex (and gender). Some people have more than just a pair, but the presence of a single “X” or “Y” is all that matters.

Dos Equis represents more than just the name of a Mexican beer. XX = Female. XY = Male. Y is NOT (just) a little X with a broken-off or missing leg! Y is mainly the result of testosterone. Why? I don’t know. The more testosterone a man has the less he likes to be asked questions. Ironically, the Y chromosome is smaller than the X chromosome – but the body it produces (with it’s X-partner) is usually larger.

Having more testosterone does many things. One thing more testosterone does not do is increase the size of a man’s penis. Women are sometimes accused of having penis envy? What about clitoris envy? The only known “purpose” of a clitoris is “pleasure”. Perhaps this is why more men seek sex change operations than women. Ironically, what they lose may be more than they gain – especially when it comes to their genitalia. Men also seem to lose more than women when their relationships break up. Women are usually far better (and faster) at “recovering” and “moving” on than men. Most women do not know this – or that men usually fall in love faster and harder than they do (which may be one reason they often seem to resist doing so).

MOST human behavior not having to do with eating, sleeping, and simply staying alive tends to be related (in one way or another) to meeting, greeting, dating, mating – and expressing the X in their sex.

Although the “purpose” of life may be more than simply survive and replicate, male and female are in many ways simply other names for impregnator and gestator. Males and females are more different than they really need to be to simply play their different roles in reproduction – and sexuality is not just restricted to the loins; it permeates the whole body. In every human society, men and women have had different roles and different destinies. The same was true for animals they kept as pets and herded or hunted for food.

The sex of a child is determined at the moment of conception by whether its father contributes an X sperm or a Y sperm to its creation. Although it may make it seem like mothers have no influence in the sex of their children, her egg actually “chooses” which sperm to allow in. So, in a sense, the mother/egg has complete and total control over the future sex of their child and the father/sperm has little/none. X sperm are slightly larger than Y sperm, as they carry a heavier sex chromosome. Heads of X sperm are more circular than those of Y sperm. The (female) egg is the selector, not the first, fastest, or fittest (male) sperm to arrive.

The X is a bold, full size, bona fide chromosome, whereas the Y is a sad, shrunken, vestigial thing, yet men are not men because they have only one X. Occasionally babies are born who have only one X chromosome and no Y. The XO babies invariably look female, although after puberty they start to look different from XX women.

Having a Y chromosome nearly always makes someone male. Whatever else is going on in the jumble of chromosomes, the presence of at least one Y seems to be sufficient to make an obviously male child. No matter how many extra X’s there are to force the issue, a single Y can usually win through and make a child into a boy.

Testicles (and testosterone) make boys into boys, and girls become girls not because of breasts or ovaries but because they do not have testicles (or as much testosterone).

The true keeper of the keys to masculinity is a gene called Sry (an abbreviation of the rather un-poetic “sex-determining region on the Y chromosome”). Sry is a single gene that seems to control the development of a whole organ system, and eventually a whole child. The Sry protein is only made in the gonad, and only for a couple of days, and then it fades away, but that is enough. That little flutter of activity is sufficient to drive a cascade of changes that will alter the embryo’s future forever, and pretty much the same thing is now known to take place in human baby boys as well. Sry is the switch that is flicked to make a boy. Other genes that are switched on by, or interact with, Sry include Dax1, Gata4, MIS, Sf1, Sox9, Wntl, and Wt1.

Making boys is a full-time job for Sry, but it is only a part-time job for Sox9. Sox9 t spends most of its time doing other things (that are not really understood). Chromosome -> Sry -> Sox9 -> MIS -> testicle -> ? -> ? -> little boy.

Testicles make boys by releasing hormones. Hormones are just chemicals made by one part of the body, which are then cast adrift in the blood to make landfall in some different, distant part of the body. Perhaps the first hormone to start to mold an XY embryo into a boy is the rather inelegantly titled Mullerian inhibiting substance (made by that MIS gene).

Boys need a thin tube to carry sperm from the testicles to the penis (the vas deferens), but girls need much wider-bore tubing to make their reproductive organs (Fallopian tubes, uterus, and vagina).

Before six weeks of development we all have the forerunners of sperm tubes, Fallopian tubes, and a uterus. If anything, we are not sexless, but double-sexed. To become a boy or girl, all we have to do is discard the set of tubes we do not require. Men do not need a uterus and this hormone [MIS] is what rids them of it.

Eggs and sperm develop from the same primordial germ cells in the indifferent gonad, and it is the “sexualization” of that gonad that actually tells them what to become. Two X chromosomes -> no Y -> no Sry -> construction of ovary.

All embryos start off with two sets of tubes – the male Wolffian ducts and the female Mullerian ducts – and the testicles actively encourages the former and destroys the latter.

The ovary is remarkable not for what it does, but for what it does not do.

Female as-default is a challenging idea because it is politically incorrect but scientific incontrovertible, at least superficially. Human babies are initially made female, but can be modified into males. One is the original and one is derived.

Scientists no longer believe that the human race is the pinnacle of creation – or that reproductive creation requires the involvement of more than one individual.

The most obstinate celibate of the animal kingdom are animals that either just split into two by binary fission, like the microscopic Amoeba, or neatly sprout new individuals out of their sides, like Hydra, another favorite of school biology teachers. The other asexual creatures are those that procreate by virgin birth, so-called “parthenogenotes.” Parthonogenetic species are entirely female, and each female can produce identical daughters without the need for any input from a male animal – they simply conceive spontaneously. The best known of these ultra-matriarchal species is probably the aphid, but examples closer to home include species of lizard and snake that breed by virgin birth.

There are many sexual species that cannot be subdivided into neat male and female populations, but perhaps the best examples of plasticity of sexuality are hermaphrodites. Hermaphrodites are animals that posses both ovarian and testicular material at some point in their life. They reproduce sexually, but they do not have clear-cut, mutually exclusive sexes. There are also sequential hermaphrodites and synchronous hermaphrodites.

Baby crocodiles and turtles are hardly in a rush to breed; except for humans, parrots, and whales, they are probably the longest-lived vertebrates in existence. They have 50 to 150 years to make babies.

Drone’s chromosomes are not paired, so they have half as many of them as workers and queens. In fact, it is this reduced number of chromosomes that actually makes them male. Hence, in bees it is the number of sets of chromosomes that decides the sex of offspring and not the presence of any particular gene.

The human Y is the arbirter of sexuality, but in flies it is the number of X chromosomes that a maggot receives which decides its sex.

Human: XX, female; XY, male; XXY, male; X-, female
Fly: XX, female; XY, male; XXY, female; X-, male

It is the number of X chromosomes that decides the sex of a baby fly.

In poultry, cocks have a ZZ chromosome pair, and hens have a ZW chromosome pair.

The Y chromosome is really just a vehicle to carry the Sry gene. Apart from Sry, the Y chromosome has become almost useless.

Hemophilia can often be treated with injections of factor VIII

“Dystrophy” means “bad nourishment”. Muscular dystrophy is much more of a strictly female-carrier, male sufferer disease.

Color blindness is much rarer in women than men.

Our primate ancestors evolved three-color vision to find fresh young leaves.

Homosexuality may result from an interaction between a male baby’s Y chromosome and his mother’s immune system. Homosexual men with maternal uncles and male cousins were more likely to be homosexual than the general population, whereas their fathers and paternal uncles and male cousins were not.

One of the genes on the Y carries the code for a protein called H-Y, and H-Y is often recognized as foreign by women’s immune systems.

Boys with older brothers are likely to have their male brain development altered by their mother’s antibodies. Each of her cells contains two X’s, but only one functional one. No matter how many supernumerary X chromosomes are stuck in into cells, inactivated X chromosome Xist reliably switches on in all except one. Women are left with the same number of functional X chromosomes as men – one. So, in short, inactivation has a very important function: to protect women from the toxic effects of their own chromosomes.

Identical twin girls are neither as rare nor as identical as identical twin boys.

More female “monozygotic” twin pairs (derived from a single fertilized egg) are born than male. This seems to be linked to the point at which the developing embryo splits into two.

Approximately one-twentieth of a girl’s genes reside on a X chromosome – so identical twin girls can potentially differ by up to 5 percent in the genes they use just because they use their X chromosomes differently.

© 2010 – 2015, Oren Pardes. All rights reserved.

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