How Do You Know You Have the Correct Ancestor When You Find Several People With the Same Name
by Katherine J. Wu
figures by Daniel Utter
Permit's talk about sex.
Seriously. Not intercourse, though – more than about how genetic sexual activity is programmed during development. Sexual identity has been in the news ofttimes lately, and unsurprisingly so: the past few years have yielded sweeping reforms in ceremonious rights, spurring new conflicts surrounding everything from age-old 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 well-nigh gender identity to draw whatsoever conclusions nearly its biological underpinnings, and certainly not well-nigh what is "right" or "wrong." Nosotros are only at present beginning to fully sympathise how mammalian sexual identity has evolved, and its dependence on the sexual practice determination systems that allow biological development of sexual characteristics in dissimilar organisms.
The sex determination we'll talk over today is (unfortunately?) not the dogged resolve to copulate. Nearly 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 about sexually reproducing organisms, there are two sexes – but the ways in which these sexes are determined and the means in which they manifest vary greatly. What are the ways in which sexual characteristics are encoded? Why are in that location so many systems for one seemingly common result?
SRY not SRY
We were all taught the classic recipe in grade school: an X chromosome from mom and an 10 chromosome from dad will yield a genetic female person, while an 10 chromosome from mom and a Y chromosome from dad will yield a genetic male. The XY sex decision arrangement (Effigy 1A) is certainly what'southward most familiar to us, and it'south used in well-nigh other mammals, as well as a few select insects and plants. Briefly, human being cells all carry chromosomes, which carry our genes. When egg meets sperm, each parent contributes 22 not-sex chromosomes and one sex chromosome – always an X from the female parent, and either an X or Y from the father. Thus, the contribution from the begetter determines the sex of the baby[1].
Post-obit fertilization, a fetus begins to develop. At first, its sexual organs manifest as a genderless gonad, or sexual practice gland – basically a small, thick ridge of tissue most what will become the abdomen. The "default" sex (i.eastward., without any other further input) is actually female – however, the presence of a factor called SRY on the Y chromosome initiates the release of testosterone and the formation of male person sexual activity organs. SRY is a transcription factor – a genetic element that can plough on the expression of other genes. In this way, SRY is similar the principal switch to plow 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 1 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.
It seems like a pretty clear arrangement – but information technology wouldn't be biology without exceptions and extra rules muddying the waters. When it comes to sex activity chromosomes, X's and Y'due south are not the but ingredients available. Many other sex determination systems be, and the concept of "male person" vs. "female" isn't quite every bit simple equally humans once thought.
The Birds and the Bees (and Some other Things Too)
Unsurprisingly, with the immense variation observed in our natural world, more one sex decision organization exists. Ours, XY, is not even predominant. A few central examples tend to predominate: the ZW arrangement in birds, XO in insects, haplodiploidy, and environmental sexual practice determination systems.
The ZW system (Figure 1B) exists in birds and some reptiles, and operates opposite of XY: females become the mixed fix of sex activity chromosomes (ZW), while males are ZZ. Thus, unlike in humans, the female parent's contribution determines the sex of the progeny[2]. Just equally 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 development of the offspring, which will otherwise "default" to male.
In the XO sexual activity determination system (Figure 1C), which is found in several insects, females are still Twenty, simply instead of carrying a Y chromosome, males simply carry a single X – the "O" in "XO" indicates the absence of a 2nd sex chromosome. Each sperm carries either an X chromosome or no sex chromosome at all – but once once more, every bit in XY, the father'south contribution determines the sex of the offspring.
After this, things start to get a petty weirder. Honeybees apply the system of haplodiploidy (Figure 1D), in which unfertilized eggs (which carry only 1 set of chromosomes and are thus haploid) develop into males and fertilized eggs (which bear 2 sets of chromosomes and are thus diploid) develop into females. Importantly, this is distinct from the XO system, where progeny inherit 2 copies of all not-sex chromosomes, regardless of sex activity; in haplodiploidy, males inherit merely ane copy of all chromosomes, sexual practice and non-sexual activity (Figure 2A).
Honeybee colonies typically center around a single fertile queen, serviced past an army of male drones and female person 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 take no fathers and can produce no sons. Furthermore, if a queen chooses only one drone to mate with, all her daughters volition share 75% of their genes with each other (different in humans, where siblings share fifty% of their genes) because they each inherit the full set of their male parent's genes, rather than only 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, information technology turns out to be more than evolutionarily advantageous to protect your sisters (with whom you share 75% of your genes) than information technology is to produce daughters of your own (with whom you lot share simply fifty% of your genes) (Figure 2B). Thus, the community structure revolves effectually the queen. This is an interesting case where the genetically determined sexual practice of individuals shapes their role within the larger community.
Finally, there exist systems in which sex decision isn't dependent on chromosomes at all. In alligators and some turtles, the temperature at which the egg is incubated during a sensitive menstruum determines sex: lower temperatures produce females, college temperatures produce males (the phenomenon of "cool chicks" and "hot dudes") (Figure 1E). However, this rule does not concur truthful in every species – sometimes the opposite rule is in upshot, or temperatures at either farthermost produce i sex, while an intermediate temperature produces the other. Some snails and fish are actually able to reverse sex midway through life, depending on ecology conditions, in a process called sex reversal. Thus, genetic sex is a far more fluid process than ane might presume.
The fact that genetic sex can be directed past the flip of a single switch may be surprising. Sex is complex – but so over again, in that location are a lot of other factors at play and, clearly, environment tin can take a large influence on how sex expresses itself. Additionally, there are many documented cases of humans with a genetic sexual activity that appears "contrary" to their physical appearance. For instance, nosotros know of genetically XX persons who have developed testes and external characteristics of men, and genetic XYs who develop every bit females. An example of the latter case occurs in Swyer Syndrome, often when there is a mutation in the SRY factor. 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 do non get signals to go testes. Swyer Syndrome patients develop externally as female, simply do not have 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 sexual activity chromosome is missing, oftentimes leading to developmental defects. Thus, all it takes is a small genetic change to turn SRY, or any of the genes information technology targets, on (or off).
(De)Generation Y
We know very little about how sexual reproduction and sexual activity decision systems evolved – the theories are, of form, difficult to test. Merely another important question is, one time sexual reproduction did evolve, why did it branch off in then many ways? And, perhaps more pressingly, is it still evolving in ways that could impact us?
The answers are notwithstanding more often than not elusive. In that location has been some indication that the XY and ZW systems are yet connected to a mutual ancestor, even though they manifested a complete reversal somewhere downwards the line. One small merely interesting line of evidence lies in the platypus, which encodes a whopping ten sex activity chromosomes (males are XYXYXYXYXY instead of XY – obviously, size matters to platypodes) that acquit great similarity to the bird Z chromosome, but technically operate under XY sex determination rules. Interestingly, though, the platypus Y lacks SRY. Thus, platypodes may terminate upward being the "missing link" between these two systems.
Furthermore, analysis of the Y chromosome has indicated that it probably evolved from the Ten chromosome, acquiring some literal "man power" forth the way. This "differentiation" event solidly distinguished the roles of the two chromosomes, and they began to evolve away from each other over time. In its current country, the Y chromosome is much smaller than the 10 chromosome, and appears to accept lost the unnecessary X genes[three] along the way. Y continues to showroom signs of this (very, very slow) Y degeneration as time progresses. In fact, the XO sex activity decision system is believed to have arisen from complete loss of an effective Y chromosome that was ultimately discarded for its relative inefficiency. There'south no demand for panic, though, XY readers – your Y chromosome is unlikely to be going anywhere anytime shortly, or maybe ever. Complete loss of Y is a pretty extreme event, and much evidence has accumulated that the loss of genes from the Y chromosome will ultimately plateau.
Plenty of Fish (Sexes) in the Sea
Sex determination in humans is fairly well established. But our system is neither the dominant style of sex determination, nor a more than "right" version of it. A final lesson comes in with the fairly new discovery of polygenic sex determination (PSD), wherein multiple genes and chromosomes contribute to the ultimate sex activity of offspring. This tin take the form of XY and ZW systems being combined into the aforementioned species, for instance. Domesticated cantaloupes (aye, the fruit) produce four sexes, and there is some show that several species of fish rely on PSD). This organization is withal poorly understood, only importantly, the added variation on each side of the equation indicates that even genetic sex is oft not a binary trait. Perhaps it'south time to rethink our preconceptions about the divides between "male" and "female."
Katherine Wu is a 3rd-year graduate student in the Biological and Biomedical Sciences Programme at Harvard University and is, equally far as she knows, Twenty and not XY.
[1] … Making King Henry VIII'south decision 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 time – no one did. Then, moot point.
[2] If but Anne Boleyn had been a pigeon.
[three] For the most office, the genes on the X chromosome only demand to be present in i copy, hence the favoring of "loss" of duplicates on the Y chromosome. In fact, in women, who take two X chromosomes, one Ten chromosome in each cell is packaged into a dormant land called a Barr torso. This actually happens at random in each cell (that is, it's non e'er the X from mom or the X from dad that'due south turned off – it can be one or the other), resulting in "mosaicism." This is actually how the coats of calico cats are patterned, and why the vast majority of calicos are female! Absurd fact: if you stumble upon a male person calico cat, it is almost certainly XXY.
Further reading
- To learn more than nigh why honeybees can't produce males from fertilized eggs, check out this brief article: http://mbe.oxfordjournals.org/content/early on/2013/12/06/molbev.mst232.full
- An excellent review on the development of sex conclusion: http://journals.plos.org/plosbiology/article?id=ten.1371/journal.pbio.1001899
Featured image from Wikimedia Commons.
Source: https://sitn.hms.harvard.edu/flash/2016/im-xy-know-sex-determination-systems-101/
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