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A new genetic study has added more evidence to a decades-old association between blood type and the risk for pancreatic cancer.
Close to <a href="http://seer.cancer.gov/statfacts/html/pancreas.html" target="_blank" rel="noopener noreferrer">42,500 men and women</a> are expected to be diagnosed with pancreatic cancer in the United States this year.   Because there is no effective screening test for the disease and it is often not detected until it has spread, the five-year survival rate for people with pancreatic cancer is only about 5%. The new results, published online this week in the journal <a href="https://dx.doi.org/10.1038/ng.429" target="_blank" rel="noopener noreferrer">Nature Genetics</a>, represent one of the first associations of a common genetic variant with this deadly disease.
Researchers from Harvard and the National Cancer Institute studied more than 4,300 people with pancreatic cancer and 4,500 controls.   They found that each copy of the C version of is associated with 1.2 times increased odds of pancreatic cancer compared to two Ts at this SNP.   The study subjects were mainly of European descent, but this association was essentially the same when the small number of people from other ethnic groups (mainly Asian and African American) was also included in the analysis.
The non-risk T version of this SNP is linked to the variation in the ABO gene that results in type O blood.   People with two Ts are type O.   The riskier C version of the SNP is found in people with type A, B and AB blood.
Epidemiological studies from the 1950s and 1960s, as well as more <a href="https://www.ncbi.nlm.nih.gov/pubmed/19276450" target="_blank" rel="noopener noreferrer">recent reports</a>, have suggested that people with type O blood are at lower risk for pancreatic cancer compared to those with type A or type B blood.   Finding the association of with the disease in a genetic study adds more credence to these previous results. What&#8217;s still not understood, however, is why people with type O blood are less likely to get pancreatic cancer.
In their concluding remarks, the authors of the study stressed the need for further research into the genetic risk factors for cancer of the pancreas.
&#8220;As there are few known risk factors, improved diagnostics and a finer understanding of the molecular pathogenesis are urgently needed…. The discovery of additional genetic risk variants for this highly lethal cancer could contribute to improvements in … prevention, early detection and therapeutic approaches to pancreatic cancer,&#8221; the authors write.

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23andMe Blog Team

Genomewide Study Supports Blood Type As A Risk Factor For Pancreatic Cancer

&#8220;But we always go there!&#8221;
And so begins another Friday night. When it comes to choosing where to go to dinner, my husband likes to stick with the tried and true. I like trying out new places.
A new study suggests that the roots of this conflict could spring partly from our genes. It suggests that a DNA variation affecting the neurotransmitter dopamine influence a person&#8217;s willingness to explore new options instead of sticking with the status quo.
<h6>Trying Something New</h6>
The finding comes from a study by Michael Frank and colleagues from Brown University and the University of Arizona. The researchers focused on how people learn from positive and negative feedback. Subjects were confronted with a clock face that counted down five seconds.   Before time was up they had to push a button to receive points.   In some trials, the experiment was set up so that the faster they pushed the button, the more points they got. In other trials, waiting longer got more points.
To the researchers&#8217; surprise, people showed wide swings in response speed within each type of trial as they adjusted their timing in an attempt to maximize their scores. Computer models showed that a likely reason for these swings is that people change their strategy (pressing the button faster or slower) in proportion to how uncertain they are that a new strategy (speeding up or slowing down from what they&#8217;ve been doing) will yield better results.
It makes sense: If you think a new restaurant might be only marginally better than the one you usually go to (and could be worse), you&#8217;re probably not that likely to vary from the usual routine. Why risk it?
But if you really have no idea how good a place might be — who knows, it could blow your mind – you&#8217;d probably be more inclined to give it a whirl.
<h6>Adventurousness</h6>
Further analysis of the data, which will appear in the August issue of <a href="https://dx.doi.org/10.1038/nn.2342" target="_blank" rel="noopener noreferrer">Nature Neuroscience</a>, showed that the extent to which a person tried out new strategies correlated with variations in the COMT gene. People who carried the &#8220;Met&#8221; version of the gene were more exploratory in the face of uncertainty about what strategy to try next than people with two copies of the &#8220;Val&#8221; version (&#8220;Met&#8221; and &#8220;Val&#8221; refer to particular amino acids encoded by different versions of the gene).
People with two copies of the Met version were the most adventurous, but even those with only one copy were statistically different in their exploration of new strategies from the people with two copies of the Val version.
The protein encoded by the COMT gene is involved in dopamine signaling in the prefrontal cortex, an area of the brain involved in planning and decision-making.   The Met version of the gene leads to increased dopamine activity in this region and has been linked to more efficient information processing.
So does this explain my date-night drama?   Well, there&#8217;s undoubtedly more to it than genes alone, but I do have one copy of the more exploratory Met version of the COMT gene.   And my husband?   Two copies of the stuck-in-a-rut Val version.
<h6>Dopamine and Learning</h6>
In a region of the brain called the basal ganglia, dopamine helps us internalize positive and negative feedback in order to develop those &#8220;gut&#8221; feelings of what strategy will work and what won&#8217;t.
The effects of dopamine in the basal ganglia have been shown in experiments that use drugs to raise or lower levels of the neurotransmitter in the brain.   Higher dopamine levels help people learn to repeat rewarding behaviors, while lower dopamine leads to better learning from bad experiences.   In a game where &#8220;A&#8221; usually yields more points than &#8220;B,&#8221; people with boosted dopamine levels learn to choose A.   People with decreased dopamine levels learn to avoid B.
In non-medicated test subjects, genetic variations that influence dopamine signaling in the basal ganglia also impact so-called &#8220;Go&#8221; (choose A) and &#8220;NoGo&#8221; learning (avoid B). People with two copies of the A version of a variant in the DARPP-32 gene, which increases dopamine signaling, tend to be better at Go learning than their G-version-carrying friends.   
The clock-and-button experiments Frank et al. conducted further tested the association of these two variants with Go and NoGo learning.   Trials that rewarded faster responses measured Go learning.   Trials that rewarded holding off on the action of button pushing measured NoGo learning. 
<h6>Parkinson&#8217;s Disease Connection</h6>
Understanding the role of dopamine in learning from experience may have important implications for treating people with Parkinson&#8217;s disease, which is characterized by a loss of dopamine producing neurons in the brain.   Studies have shown that people with Parkinson&#8217;s have trouble with Go learning.   It&#8217;s thought that the lack of dopamine in their brains prevents the dopamine spikes needed to learn from positive feedback.
This fits with evidence that drugs that increase dopamine help people with Parkinson&#8217;s improve their performance on tasks that require Go learning.   But there is a downside:   because they flood the brain with dopamine, the normal dips in signaling that are needed to learn from negative feedback are blocked by these drugs.   This might explain why some people with Parkinson&#8217;s disease who take dopamine-increasing medications develop gambling problems — they&#8217;re overly attuned to winning, but incapable of learning from their losses.

DNA Variation May Help Us Break Free From Our Routines

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Ten millennia ago, there were about six million people on Earth. Today, there are six billion.
This thousandfold increase in the global population is often thought to be linked to the invention of farming and the domestication of animals about 13,000 years ago in the Near East. Growing crops and raising live animals requires a larger work force than hunting or gathering, so as agriculture took hold, families grew in size. These families grew into villages, villages grew into towns and cities, which eventually consolidated into the vast civilizations in ancient Mesopotamia, Egypt, and elsewhere.
Was this surge during the early days of agriculture the ONLY time in the history of our species that the worldwide population size of humans experienced a substantial boost? The traditional answer was yes. Early human hunter-gatherer populations were relatively stable, their overall size not changing much from generation to generation. Stone-age humans had a limited number of gazelles to hunt, berries to pick and eggs to snatch. Those ecological limitations suggest that pre-agricultural communities would have grown only modestly, even when the hunting was good.
But a new DNA analysis published in the July 29 issue of <a id="a:sf" title="PLoS One" href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0006366" target="_blank" rel="noopener">PLoS One</a> suggests that the world&#8217;s human population grew dramatically even long before the development of farming.

Earlier analyses of mitochondrial DNA (mtDNA) have suggested that there were bursts of population growth much further back in time, but those studies have been unable to pinpoint a specific time span.
Geneticist Michael Hammer and his colleagues looked at nuclear DNA from four African populations: the San hunter-gatherers of the Kalahari Desert, the Biaka Pygmies of the Central African Republic, the Mandenka of Senegal and the Yorubans of Nigeria. Nuclear DNA is useful here because it is more sensitive to population size changes than mtDNA.
The authors found genetic evidence for a surge in human population size about 40,000 to 50,000 years ago. This period, just after humans first set foot outside Africa, is of great interest to archaeologists because it coincides with a dramatic increase in the sophistication of human behavior. People began crafting tools from bone, burying their dead and fashioning clothing to keep themselves warm in cool climates. They developed complex hunting techniques, and created great works of art in the form of cave paintings and jewelery.
The archaeological record also shows that during this time, humans began hunting more dangerous prey and more easily exploiting small game like rabbits and birds. They traveled farther than they had before, perhaps due to the growth of long-distance trade routes &#8211; the first of their kind. Jared Diamond, author of The Third Chimpanzee, calls this period &#8220;The Great Leap Forward,&#8221; when humans burst forth culturally &#8211; finally separating themselves from their evolutionary cousins.
The exact cause for these changes in human behavior may never be known. Some believe a simple genetic mutation or that the evolution of language could have sparked such a dramatic change. But what we do know now, thanks to this new genetic research, is that like the invention of agriculture this explosion of innovation was accompanied by population growth.

The First Population Explosion: Human Numbers Expanded Dramatically Millennia Before Agriculture

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Mutations are bad, right?
Not always.  Some DNA changes are completely neutral. That&#8217;s how the human genome came to have so many variations. And some mutations are actually advantageous.
A case in point is the PCSK9 gene. So-called &#8220;loss-of-function&#8221; mutations that prevent the protein encoded by this gene from functioning properly actually lead to lower cholesterol levels.
Researchers at several pharmaceutical companies have taken inspiration from these PCSK9 mutations. They are now developing drugs that would block its function in people with non-mutated forms of the gene.   These drugs, though still in the early stages of testing, may offer a new way for the tens of millions of Americans with <a href="https://www.23andme.com/topics/health-predispositions/fh/">high cholesterol</a> to get their levels under control.
The PCSK9 protein binds to LDL cholesterol receptors on the surface of cells.   Once cholesterol binds to the receptor too, the whole complex is internalized into the cell, and the receptor is degraded.   When PCSK9 function is missing, the LDL receptor is able to recycle back to the cell&#8217;s surface after dropping off its cholesterol cargo in the cells, allowing it to sop up more &#8220;bad&#8221; cholesterol from the bloodstream.   There doesn&#8217;t seem to be any downside to the cholesterol-lowering PCSK9 mutations, suggesting that targeting the protein with drugs could be safe and effective way of reducing cholesterol.
One company, <a href="https://dx.doi.org/10.1073/pnas.0903849106" target="_blank" rel="noopener noreferrer">Amgen</a>, is tackling the problem with antibodies that attach to the PCSK9 protein outside of cells, targeting it for destruction by the immune system.   Two others, Isis and <a href="https://dx.doi.org/10.1073/pnas.0805434105" target="_blank" rel="noopener noreferrer">Alnylam</a>, are using small pieces of RNA designed to go inside cells and keep the PCSK9 protein from being made in the first place.   All three companies have shown reductions of LDL cholesterol in early animal experiments.   A review of their findings appeared recently in Nature Publishing Group&#8217;s <a href="https://dx.doi.org/10.1038/scibx.2009.895" target="_blank" rel="noopener noreferrer">Science-Business eXchange</a>.
People who need to lower their cholesterol but can&#8217;t tolerate statins stand to benefit most from any PCSK9 inhibitors that are developed.   But these drugs might be good for those taking statins too.   High doses of statins can actually increase the amount of PCSK9 in the body.   Taking a PCSK9 inhibitor along with a statin could help cut off this potentially backtracking side effect.   Unfortunately, unlike statins, none of the PCSK9 inhibitors under development can be taken orally.   They all need to be delivered through an injection.
Many years of clinical trials are ahead for each of these drugs.   Researchers will need to assess both their safety and efficacy.   But it&#8217;s exciting to see a genetics discovery be so quickly translated into an idea that could help millions of people.

Genetic Research Could Lead to New Cholesterol Drugs

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In 1974, scientists digging in the dry lake bed of Lake Mungo in southeastern Australia uncovered the skeleton of a man preserved in the deep layers of sand and clay. Dating techniques eventually revealed that this individual died about 40,000 years ago.
Scientists and the popular press dubbed the individual &#8220;Mungo Man.&#8221; Why did he make such a splash?   Not only because he was &#8211; and remains &#8211; one of the oldest and most complete skeletons of the earliest Australians, but because his appearance shattered the previously held notion that humans had first set foot in Australia less than 10,000 years ago. It was so far from where humans arose in Africa, and so remote.   So of course humans arrived there so much later than everywhere else, many experts reasoned. With the discovery of Mungo Man, this idea lost support, and scientists now concede that Australia was settled much earlier than many other parts of the world, including the Americas and parts of Europe.

While this discovery initially answered many questions regarding the peopling of Australia, it left many more unanswered – especially how people could have reached an island continent so soon after humans first expanded beyond Africa about 60,000 years ago.
The thinking is that after leaving Africa, one or more groups of humans journeyed from their homeland in East Africa into Arabia via the Red Sea. Over the next several thousand years, their descendants continued along the coasts of Arabia and India, eventually heading south into present-day Indonesia and finally to Australia, which was joined with the island of New Guinea at the time.
There has been some archaeological and genetic evidence of such a migration, but most of it has been indirect or circumstantial. Some scientists remain unconvinced because researchers have not been able to show a direct link between modern Australian Aboriginals and modern people living along the coastal route from Africa. But now, in a new study led by the Anthropological Survey of India, geneticists believe they&#8217;ve found the first concrete evidence of such a link. Their results are reported in the July 21 issue of <a id="t6n." title="BMC Evolutionary Biology" href="http://www.biomedcentral.com/1471-2148/9/173/abstract" target="_blank" rel="noopener">BMC Evolutionary Biology</a>.
The team, led by Satish Kumar, reasoned that if the hypothesis of an ancient migration along the Indian Ocean coast toward Australia was accurate, there would be evidence in the DNA of modern people living along that path. So they compared the DNA of modern Australian Aboriginals to that of tribes from India, such as the Baiga of central India and the Birhor of eastern India. These groups are often called &#8220;relic populations&#8221; because they are believed to share many cultural, linguistic, physical and genetic features with the region&#8217;s ancient inhabitants.
Experts have long noticed that the Baiga, Birhor and other relic populations share physical similarities with native Australians. Kumar and his team reasoned that there could be DNA similarities too.
Kumar led the extraction and analysis of mitochondrial DNA (mtDNA) from nearly 1,000 individuals from Indian relic populations. For comparison, they used Australian Aboriginal DNA data that had already been analyzed and published by colleagues. After comparing the two groups, they came to a startling conclusion: two specific genetic mutations on the mtDNA of the Indian and aboriginal samples matched perfectly. Not only that, but these particular mutations do not exist elsewhere in the world; they are shared exclusively between a few isolated tribes in India and native Australians.
Kumar and his colleagues concluded the two groups must share a common ancestry. To lend further credence to their theory, they calculated the date when the ancestors of the Indian tribes and Australian Aboriginals must have split.
Their calculations produced a date of 55,000 years ago, a time when early humans in India were probably hunting wildlife and gathering plant foods. Some of their descendants eventually formed tribes like the Baiga and Birhor; others moved eastward, traversing southeastern Asian and then using maritime technology to cross nearly 60 miles of open ocean between Indonesia and New Guinea.
After arriving in Australia, they moved into the heart of the Australian Outback. A few thousand years later, a direct descendant of these ancient explorers was laid to rest along the shores of Lake Mungo.

Direct Genetic Link between Australia and India Provides New Insight into the Origins of Australian Aboriginals

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Researchers  have identified a <a href="https://www.23andme.com/dna-health-ancestry" target="_blank" rel="noopener">genetic variant</a> that increases risk of follicular lymphoma (FL), a type of blood cancer.
FL is a type of Non-Hodgkin lymphoma (NHL), the fifth most common cancer in the U.S. Nearly 20,000 patients die and 66,000 more are diagnosed with NHL each year. The incidence of FL has doubled within the past 30 years.
In a study by Skibola et al., scientists analyzed the genomes of 4,805 people of European descent from the United States, Germany and Canada to search for genetic associations with four different subtypes of lymphoma.  The results, published online this week in the journal <a href="https://www.nature.com/ng/journal/vaop/ncurrent/abs/ng.419.html" target="_blank" rel="noopener">Nature Genetics</a>, newly linked one SNP to an elevated risk for developing FL in European populations.
FL is a slow-progressing cancer of the white blood cells that affects middle-aged and elderly people. It is usually diagnosed in advanced stages, but patients live approximately 10 to 12 years after diagnosis.
Skibola et al. revealed that having one copy of the C version of a SNP located on chromosome 6, is associated with 1.69 times increased odds of FL compared to people with two copies of an A at this SNP. Carrying two copies of C at this SNP is associated with 2.22 times increased odds of FL. Because a majority of the population carries the C version, another way to look at these results is to say that those who have the A version of this SNP appear to be somewhat protected from FL.*
This SNP is located near a region that is associated with susceptibility to psoriasis. The researchers said that further studies are required to evaluate whether is also associated with psoriasis due to this genetic overlap. Future research may also determine whether FL is associated with exposure to viruses such as HIV.

Genetic Variant May Increase Risk of Follicular Lymphoma

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The Neanderthals have always held a special place in the field of anthropology. &nbsp; The skeletal remains of our short, stocky evolutionary relatives have been found everywhere from Spain to Iraq.
Their physical likeness to our own species, and the possibility that humans and Neanderthals may have interacted, has long fascinated experts and enthusiastic novices alike. &nbsp; But simply studying their skeletal remains and artifacts seemed to leave more questions than answers.
More than 10 years ago an international team of scientists became the first to extract and analyze ancient DNA (aDNA) from a Neanderthal skeleton. By examining the mitochondrial DNA (mtDNA) – which is more abundant in our cells than our nuclear DNA and therefore more likely to preserve – they found that there were enough genetic differences between this Neanderthal and modern humans to classify the two as separate species.
Since this initial foray into Neanderthal genetics there have been attempts to improve aDNA analysis, with the goal of filling in the gaps that traditional anthropological techniques had been unable to. &nbsp; Earlier this year, scientists at the Max Planck Institute for Evolutionary Anthropology became the first to sequence the entire Neanderthal mitochondrial genome &#8211; no easy feat. &nbsp;Now scientists have taken aDNA analysis to the next level by developing a novel technique to extract it more easily, yielding the most comprehensive and accurate results to date. &nbsp; Along the way, they have uncovered some intriguing clues to the possible fate of the Neanderthals. &nbsp;Their results are reported in the July 17 issue of <a id="rvka" title="Science" href="http://www.sciencemag.org/cgi/content/abstract/325/5938/318" target="_blank" rel="noopener noreferrer">Science</a>.

This new study, also led by Max Planck Institute scientists, centers around a novel method for finding and extracting that elusive aDNA from Neanderthal remains. The study took advantage of a new kind of aDNA extraction process, called Primer Extension Capture (PEC). This technique has many advantages over the others, primarily because it allows the aDNA to be completely isolated from all the other molecular junk &nbsp; that can accumulate over time. This yields highly accurate results with much less effort. So instead of simply using this technique on one Neanderthal individual, they analyzed five.
The remains they chose had been excavated from a variety of locations, from Croatia to Germany to Spain and Russia. &nbsp; Most of the remains were between 35,000 and 40,000 years old, which is very close to when Neanderthals were believed to have disappeared from most of their range. &nbsp; They also examined Neanderthal remains from Russia that dated to between 60,000 and 70,000 years old.
After successfully extracting and analyzing the DNA of these remains, the researchers came to a few startling conclusions. &nbsp; First, the level of genetic diversity among the samples was exceedingly low. &nbsp; In fact, the amount of genetic diversity of the Neanderthal samples was less than one-third the diversity we see in modern humans today. For example, two of the samples &#8212; one from Croatia and the other from Germany &#8212; had identical mtDNA genomes. For two individuals living nearly 1,000 miles apart, this is quite unusual; unless there wasn&#8217;t much variation in mtDNA in the first place.
The authors of this report think this genetic homogeneity means that there were far fewer Neanderthals living in Europe and western Asia than they&#8217;d previously thought. Based on their analysis of the five individuals, the they estimated that the total population size of Neanderthals in Europe 35,000 years ago may have had as few as 3,500 females (because mtDNA is passed down maternally, it cannot be used to estimate male population size).
The authors believe there are two possible explanations for this small population size. &nbsp; The first is that, over the 400,000 year history of Neanderthals, their population may have always been small. &nbsp; After all, for much of their existence they survived harsh ice age conditions, so low population size may have been necessary for survival.
But the authors offer an alternate explanation as well. They think this small population size is the result of long-term population decline, perhaps beginning about 40,000 years ago and continuing until Neanderthals were wiped out. To test their hypothesis, the researchers re-analyzed the Neanderthal mtDNA genomes and found that their protein-encoding genes had evolved much more quickly than those of humans or chimpanzees since the three species split, millions of years ago.
This high rate of evolution, the authors argue, was not present in the older Russian Neanderthal sample. &nbsp; This fact implies a pattern of decline in population size, not a population that had been small from the start. These results support their idea that the Neanderthal numbers were on the decline. &nbsp; Without direct evidence, they can only speculate as to the cause of of their decline, but the scientists believe it may be tied to the arrival of humans in Europe and western Asia about 40,000 years ago.
Today most experts believe the Neanderthals were being out-competed by the incoming humans, who had superior technology and language skills. &nbsp; Over time, the Neanderthals were forced to move to more isolated regions in the mountains of France and Spain. &nbsp; By 30,000 years ago, only a few traces remained. &nbsp; Soon after, they were gone. &nbsp; This study reveals some compelling evidence that humans were in fact responsible &#8211; whether directly or indirectly &#8211; for the demise of the Neanderthals.

Novel Techniques Suggest Neanderthal Populations Dwindled in the Face of Expanding Humans

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Researchers have identified a genetic variant that increases risk of atrial fibrillation (AF), an electrical disorder of the heart, which in turn increases risk of stroke.
AF affects one in four people over 40 years old, and stroke is the third leading cause of death in the United States.
In one of two studies, scientists scanned the genomes of 5,806 people from the United States, Iceland and Norway to look for genetic variations associated with AF. The other study combined data from several large studies comprising almost 47,000 people of European ancestry.

Their results, published online this month in Nature Genetics, newly linked one SNP to slightly elevated risk for developing AF in European populations.
Most strokes are caused by a blood clot that blocks circulation to the brain. Because AF causes ineffective beating of the heart, blood can pool and clot in the upper chambers, resulting in stroke if a clot leaves the heart and lodges in the brain. AF increases the risk of stroke four to five times across all age groups and is the cause behind 10 to 15 percent of all blood clot-related strokes.
In the first study, Gudbjartsson et al. studied people of European descent from the United States, Iceland and Norway to reveal that the T version of   a SNP on chromosome 16, is associated with 1.21 times increased odds of AF. This SNP was also associated with 1.11 times increased odds of ischemic stroke and 1.22 increased odds for cardioembolic stroke.
In the second study, Benjamin et al. associated AF with another SNP. Both SNPs are likely to be marking a third genetic variation that actually has a functional effect on AF. Further research will be necessary to identify both the variation and its effect.
Gudbjartsson et al. also assessed the association of AF with the T version of in more than 3,000 Han Chinese, but the association was not significant. They noted that the T allele of this SNP is much more frequent in Han Chinese than in European populations.

Genetic Variation May Increase Risk of Atrial Fibrillation And Stroke

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It may be you&#8217;ve heard a rumor that males are on the brink of extinction.
Whatever you may think of that prospect, the rumor is false. But over the past decade, numerous <a href="https://www.ncbi.nlm.nih.gov/pubmed/17400006?ordinalpos=1&amp;itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DiscoveryPanel.Pubmed_Discovery_RA&amp;linkpos=1&amp;log$=relatedreviews&amp;logdbfrom=pubmed" target="_blank" rel="noopener">studies</a> have hinted that the Y chromosome, a male necessity, is going the way of the dodo.
Though other studies have suggested this idea may be a bit of an exaggeration, a new report this week suggests that the Y chromosome may indeed be endangered.
In most mammals, such as us humans, two chromosomes determine the sex of each individual organism: &nbsp; the X and the Y. &nbsp; If an individual&#8217;s cells contain two copies of the X chromosome, then they will be genetically female. &nbsp; If they contain one copy of the X and another of the Y, they will be male.

Yet even though these aptly named sex chromosomes have a similar duty – to confer sex – the X and the Y could not be more different. &nbsp; The most striking difference between the two is their size; the Y is less than half as big as the X, and contains only 78 genes, compared to the more than 2,000 found along the X chromosome. &nbsp; The evolutionary history of the two sex chromosomes and the question as to why they are so different from each other has been the subject of heated debate for many years. &nbsp; Now scientists at Pennsylvania State University believe they&#8217;ve found a way to uncover not only the difference between the X and Y, but how and why it arose and what this means for the future of the small, but essential, Y chromosome. &nbsp; Their results are reported in the July 17 issue of <a href="http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1000568" target="_blank" rel="noopener">PLOS Genetics</a>.
The research team, led by biologists Kateryna Makova and Melissa Wilson, believes the key to understanding the origins and future of the Y chromosome lie in some of our most distant mammalian relatives. &nbsp; There are three classes of mammals: egg-layers like the platypus, marsupials like the kangaroo, and the eutherians, which includes humans and thousands of other similar species. &nbsp; While there are many differences between the three groups, one of the most striking is the difference in the organization of the sex chromosomes. &nbsp; As Makova explained, &#8220;In eutherian mammals, the sex chromosomes contain an additional region of DNA whereas, in the marsupials and egg layers, this additional region of DNA [is not on a distinct chromosome, but] is [a region of] the non-sex chromosomes.&#8221;
The authors argue that the key to the origins of the X and the Y chromosomes may lie in this fundamental difference. &nbsp; By analyzing the X and Y of humans compared to the sex-determining regions of marsupials and egg-laying mammals, Makova and Wilson found that the X and Y split from the other chromosomes about 80 to 130 million years ago.
But that is not all they found. &nbsp; The authors also examined how fast the X and Y mutated over time, and noticed a startling change that occurred at about the same time as the split. &nbsp; &#8220;Our research revealed that the Y-specific DNA began to evolve rapidly at the same time that the DNA region split into two entities, while the X-specific DNA maintained the same evolutionary rate as it had previously,&#8221; Makova explained.
In other words, as soon as the X and the Y split their own distinct chromosomes, the Y began to evolve much more quickly than its counterpart, mutating at a much higher rate with each new generation. &nbsp; The faster the Y evolved, the faster its genes disappeared. &nbsp; Whereas at one point the Y may have contained thousands of genes, that number has dwindled to the mere 78.
The disappearance of these genes over time and the small number of those remaining on the Y begs the question: &nbsp; will the Y chromosome ever disappear entirely? &nbsp; The authors believed this was an important question to answer as well, and began additional analysis to determine the fate of the Y.
Makova and Wilson reasoned that there must be some utility to the Y, or else it surely would have disappeared by now. &nbsp; As Wilson states, &#8220;we know that a few of the genes on the Y chromosome are important, such as the ones involved in the formation of sperm &#8230; . &nbsp; Although there is evidence that the Y chromosome is still degrading, some of the surviving genes on the Y chromosome may be essential.&#8221; &nbsp; By performing additional tests, they found that there were indeed some genes on the Y that will probably never disappear entirely. &nbsp; But they also found several genes that were already disappearing, and were likely to be gone many generations from now.
Some recent studies have produced evidence that the genes on the Y may not be disappearing as fast as was initially thought. But, according to Makova, the Y chromosome may not be out of the woods just yet. &#8220;We still think there is a chance that the Y chromosome eventually could disappear,&#8221; she says. &nbsp; &#8220;[But] if this happens, it won&#8217;t be the end of males.&#8221; &nbsp; Instead, she believes that a new pair of sex chromosomes will arise from the genome, latching on to those few remaining genes, keeping alive the genes necessary for male survival.

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