You Know What Im Just Going to Say It

past Katherine J. Wu
figures by Daniel Utter

Let'due south talk about sex activity.

Seriously. Not intercourse, though – more about how genetic sex is programmed during evolution. Sexual identity has been in the news often lately, and unsurprisingly so: the by few years have yielded sweeping reforms in civil rights, spurring new conflicts surrounding everything from age-onetime battles in gender equality to legislation enforcing anti-transgender bathrooms. It's a complicated subject, to say the least. With regards to science, we don't know enough about gender identity to draw any conclusions almost 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 conclusion systems that allow biological evolution of sexual characteristics in unlike organisms.

The sexual practice conclusion we'll discuss today is (unfortunately?) not the dogged resolve to copulate. Most multicellular organisms, humans included, use sexual reproduction to reproduce. Compared to asexual reproduction, in which cells can merely create carbon copies of themselves, sexual reproduction allows for the introduction of genetic diversity into a population. In most sexually reproducing organisms, there are ii sexes – only the ways in which these sexes are adamant and the means in which they manifest vary profoundly. What are the ways in which sexual characteristics are encoded? Why are there so many systems for one seemingly common result?

SRY non SRY

We were all taught the classic recipe in class schoolhouse: 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. The XY sex conclusion organisation (Effigy 1A) is certainly what's most familiar to us, and information technology's used in most other mammals, as well as a few select insects and plants. Briefly, homo cells all carry chromosomes, which bear our genes. When egg meets sperm, each parent contributes 22 non-sex chromosomes and one sexual activity chromosome – ever an X from the mother, and either an X or Y from the father. Thus, the contribution from the father determines the sex of the baby[i].

Following fertilization, a fetus begins to develop. At outset, its sexual organs manifest equally a genderless gonad, or sex gland – basically a minor, thick ridge of tissue virtually what will become the abdomen. The "default" sex activity (i.e., without any other further input) is actually female person – all the same, the presence of a gene called SRY on the Y chromosome initiates the release of testosterone and the formation of male sex organs. SRY is a transcription factor – a genetic element that can plough on the expression of other genes. In this way, SRY is like the primary switch to turn on the suite of "male" genes in a developing organism. 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 i 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 babe develops female sexual organs.

Information technology seems similar a pretty articulate system – just it wouldn't be biology without exceptions and extra rules muddying the waters. When information technology comes to sex chromosomes, 10'due south and Y's are not the merely ingredients available. Many other sex determination systems exist, and the concept of "male" vs. "female" isn't quite as simple as humans once thought.

The Birds and the Bees (and Some Other Things Besides)

Unsurprisingly, with the immense variation observed in our natural earth, more than one sex decision organisation exists. Ours, XY, is not even predominant. A few key examples tend to predominate: the ZW system in birds, XO in insects, haplodiploidy, and environmental sex determination systems.

The ZW system (Effigy 1B) exists in birds and some reptiles, and operates contrary of XY: females get the mixed set of sex chromosomes (ZW), while males are ZZ. Thus, dissimilar in humans, the mother's contribution determines the sex of the progeny[2]. Only as the mammalian Y chromosome carries the male-determining SRY, the avian W chromosome carries like master switches FET1 and ASW, which are necessary for female evolution of the offspring, which volition otherwise "default" to male.

In the XO sex conclusion arrangement (Figure 1C), which is found in several insects, females are withal XX, but instead of carrying a Y chromosome, males simply comport a single X – the "O" in "XO" indicates the absence of a 2nd sexual activity chromosome. Each sperm carries either an X chromosome or no sex chromosome at all – only once once more, as in XY, the father's contribution determines the sex activity of the offspring.

Figure 1: 5 (of many) sex decision systems. A. XY organization In humans, females are Twenty and males are XY. B. ZW system In birds, females are ZW and males are ZZ. C. XO organization In insects, females take ii sexual activity chromosomes, but males have but ane sexual practice chromosome (while retaining two copies of all non-sex chromosomes). D. Haplodiploidy In honeybees, females over again take two sexual activity chromosomes while males have one, but in this case, males accept but 1 copy of every chromosome. Due east. Thermal regulation In some reptiles, the temperature of the surrounding environment determines the sex of the offspring.

Later on this, things first to go a little weirder. Honeybees apply the system of haplodiploidy (Figure 1D), in which unfertilized eggs (which carry only one set of chromosomes and are thus haploid) develop into males and fertilized eggs (which carry two sets of chromosomes and are thus diploid) develop into females. Importantly, this is distinct from the XO organization, where progeny inherit two copies of all non-sexual practice chromosomes, regardless of sex; in haplodiploidy, males inherit only 1 copy of all chromosomes, sex and not-sex (Figure 2A).

Honeybee colonies typically centre around a single fertile queen, serviced by an regular army of male drones and female workers. The queen lays a vast number of eggs, some of which are fertilized and develop into females. Those that remain unfertilized develop into males. Thus, in this system, males have no fathers and can produce no sons. Furthermore, if a queen chooses just one drone to mate with, all her daughters will share 75% of their genes with each other (unlike in humans, where siblings share fifty% of their genes) because they each inherit the full gear up of their father'due south genes, rather than just half. While this system seems vastly overcomplicated, it is believed to have been evolved to promote the social nature of honeybees: as a female worker, it turns out to exist more evolutionarily advantageous to protect your sisters (with whom you share 75% of your genes) than it is to produce daughters of your ain (with whom you share only l% of your genes) (Figure 2B). Thus, the community construction revolves around the queen. This is an interesting case where the genetically adamant sex of individuals shapes their role within the larger community.

Figure two: Sex determination in honeybees. A. Honeybee haplodiploidy Fertilized eggs inherit a set of chromosomes from their female parent and a gear up of chromosomes from their male parent, and are always female. Unfertilized eggs receive half their mother'south chromosomes and are always male; males have no fathers. B. Sisters before mothers Each daughter receives all her begetter's chromosomes and one-half her mother's chromosomes. Thus, sisters are more than related to 1 another (75%) than they each are to their mothers (50%).

Finally, there exist systems in which sex conclusion isn't dependent on chromosomes at all. In alligators and some turtles, the temperature at which the egg is incubated during a sensitive period determines sexual activity: lower temperatures produce females, higher temperatures produce males (the phenomenon of "absurd chicks" and "hot dudes") (Figure 1E). However, this dominion does not hold truthful in every species – sometimes the opposite rule is in effect, or temperatures at either extreme produce 1 sex, while an intermediate temperature produces the other. Some snails and fish are actually able to contrary sexual activity midway through life, depending on environmental conditions, in a process called sexual practice reversal. Thus, genetic sex is a far more than fluid process than 1 might assume.

The fact that genetic sex can be directed by the flip of a single switch may be surprising. Sex is circuitous – only then once more, there are a lot of other factors at play and, clearly, surroundings can have a big influence on how sex expresses itself. Additionally, at that place are many documented cases of humans with a genetic sex that appears "reverse" to their physical appearance. For instance, we know of genetically 20 persons who take developed testes and external characteristics of men, and genetic XYs who develop every bit females. An case of the latter case occurs in Swyer Syndrome, often when in that location is a mutation in the SRY gene. While the rest of the Y chromosome is left intact, a malfunctioning SRY means that the male "switch" is never flipped, and the indifferent gonads practise non get signals to become testes. Swyer Syndrome patients develop externally equally female, just practise not accept ovaries and are infertile.

Finally, inheriting extra or too few chromosomes can considerably alter how sex manifests. Klinefelter is a common example, equally well as Turner Syndrome (XO), where a sex chromosome is missing, often leading to developmental defects. Thus, all information technology takes is a small genetic modify to turn SRY, or any of the genes it targets, on (or off).

(De)Generation Y

We know very picayune nigh how sexual reproduction and sex determination systems evolved – the theories are, of class, hard to test. But another important question is, in one case sexual reproduction did evolve, why did information technology branch off in so many ways? And, perhaps more pressingly, is it however evolving in means that could affect us?

The answers are all the same mostly elusive. There has been some indication that the XY and ZW systems are still connected to a common ancestor, even though they manifested a complete reversal somewhere down the line. I small but interesting line of prove lies in the platypus, which encodes a whopping 10 sex chromosomes (males are XYXYXYXYXY instead of XY – obviously, size matters to platypodes) that bear great similarity to the bird Z chromosome, but technically operate under XY sex activity determination rules. Interestingly, though, the platypus Y lacks SRY. Thus, platypodes may terminate up being the "missing link" between these two systems.

Furthermore, analysis of the Y chromosome has indicated that it probably evolved from the X chromosome, acquiring some literal "man power" along the style. This "differentiation" event solidly distinguished the roles of the two chromosomes, and they began to evolve abroad from each other over fourth dimension. In its current country, the Y chromosome is much smaller than the X chromosome, and appears to have lost the unnecessary 10 genes[three] along the way. Y continues to exhibit signs of this (very, very slow) Y degeneration as time progresses. In fact, the XO sex determination organisation is believed to have arisen from complete loss of an effective Y chromosome that was ultimately discarded for its relative inefficiency. There's no need for panic, though, XY readers – your Y chromosome is unlikely to be going anywhere someday soon, or maybe e'er. Complete loss of Y is a pretty farthermost upshot, and much evidence has accumulated that the loss of genes from the Y chromosome will ultimately plateau.

Enough of Fish (Sexes) in the Ocean

Sex activity determination in humans is adequately well established. But our organization is neither the dominant style of sex activity determination, nor a more than "right" version of it. A last lesson comes in with the adequately new discovery of polygenic sex determination (PSD), wherein multiple genes and chromosomes contribute to the ultimate sex of offspring. This tin can take the form of XY and ZW systems being combined into the same species, for case. Domesticated cantaloupes (yes, the fruit) produce iv sexes, and there is some prove that several species of fish rely on PSD). This system is still poorly understood, merely importantly, the added variation on each side of the equation indicates that even genetic sexual practice is frequently not a binary trait. Peradventure it'southward fourth dimension to rethink our preconceptions about the divides between "male person" and "female person."

Katherine Wu is a tertiary-year graduate student in the Biological and Biomedical Sciences Program at Harvard University and is, as far as she knows, XX and non XY.

[1] … Making Male monarch Henry VIII's determination to behead a bunch of his wives for failing to bear him sons a little misinformed. Oops. Not that he could have known at the fourth dimension – no ane did. So, moot betoken.

[2] If simply Anne Boleyn had been a pigeon.

[iii] For the most role, the genes on the X chromosome just demand to be nowadays in one re-create, hence the favoring of "loss" of duplicates on the Y chromosome. In fact, in women, who accept two X chromosomes, ane X chromosome in each jail cell is packaged into a dormant country called a Barr trunk. This actually happens at random in each cell (that is, it's not always the X from mom or the Ten from dad that's turned off – it can be i or the other), resulting in "mosaicism." This is actually how the coats of calico cats are patterned, and why the vast bulk of calicos are female person! Cool fact: if you stumble upon a male calico cat, information technology is almost certainly XXY.

Further reading

1. To acquire more most why honeybees tin can't produce males from fertilized eggs, check out this brief commodity: http://mbe.oxfordjournals.org/content/early/2013/12/06/molbev.mst232.full
2. An fantabulous review on the evolution of sexual practice determination: http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899

Featured image from Wikimedia Eatables.

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Source: https://sitn.hms.harvard.edu/flash/2016/im-xy-know-sex-determination-systems-101/

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