It may be you’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 studies 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:  the X and the Y.  If an individual’s cells contain two copies of the X chromosome, then they will be genetically female.  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.  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.  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.  Now scientists at Pennsylvania State University believe they’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.  Their results are reported in the July 17 issue of PLOS Genetics.

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.  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.  While there are many differences between the three groups, one of the most striking is the difference in the organization of the sex chromosomes.  As Makova explained, “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.”

The authors argue that the key to the origins of the X and the Y chromosomes may lie in this fundamental difference.  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.  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.  “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,” 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.  The faster the Y evolved, the faster its genes disappeared.  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:  will the Y chromosome ever disappear entirely?  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.  As Wilson states, “we know that a few of the genes on the Y chromosome are important, such as the ones involved in the formation of sperm … .  Although there is evidence that the Y chromosome is still degrading, some of the surviving genes on the Y chromosome may be essential.”  By performing additional tests, they found that there were indeed some genes on the Y that will probably never disappear entirely.  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. “We still think there is a chance that the Y chromosome eventually could disappear,” she says.  “[But] if this happens, it won’t be the end of males.”  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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Genomewide association studies have had some success in finding DNA variants associated with increased risk for bipolar disorder.  But researchers from the MRC Centre for Neuropsychiatric Genetics and Genomics at Cardiff University in England have taken these studies a step further by looking for common functional themes running through the GWAS data. Their results, published online this month in the American Journal of Human Genetics, implicate some of the most basic biological pathways in the genesis of bipolar disorder.

Starting with the idea that the genetic variations increasing risk for a disorder are probably not randomly distributed throughout the genome, but instead are in one or more sets of related genes, Holmans et al. re-analyzed the SNP data from four previous studies that included a total of 4,387 cases of bipolar disorder and 6,209 controls. Researchers at the Children’s Hospital of Philadelphia recently took a similar approach (although the technical details of the analysis differ) for a study of Crohn’s disease.

The bipolar disorder researchers found that biological pathways involved in broad control of cellular activity were overrepresented in the genetic variations association with bipolar disorder.  Two of the pathways, hormone activity and cellular self-digestion, are known to be impacted by lithium, the major medication used to treat bipolar disorder. As always, more research will be needed to substantiate these findings, as well as to understand how they can be used to help the more millions of people worldwide dealing with bipolar disorder.

(23andMe customers can learn more in the Bipolar Disorder Research Report and the Bipolar Disorder: Preliminary Research Report.)

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Type 1 diabetes is on the rise in European children, says a new report.

Researchers studied type 1 diabetes data collected between 1989 and 2003 at 20 centers in 17 European countries. Their results, published online yesterday in the Lancet, show that more children, especially younger children, are being diagnosed with the disease each year.  Based on the trends they saw, the scientists calculate that there were 94,000 kids under the age of 15 with type 1 diabetes in Europe in 2005, and that by 2020 that number will soar to 160,000.

While researchers aren’t exactly sure why this is, they do know that it’s not due to changes in the prevalence of susceptibility genes.  Genes just don’t change that quickly.

An almost 70% increase in disease prevalence in one generation must be due to changes in non-genetic factors. Most random genetic changes in a population come and go pretty quickly, especially mutations that reduce fitness.  And if a new mutation does manage to stick, it would take millions of years, not tens of years, to see its effects.  Even for mutations that provide a benefit, like the one that led to the lactose tolerance seen in many people with European ancestry today, it takes a few hundred years to build-up to high enough levels in the population to cause an observable change in a trait.

An increase in disease incidence due to changes in non-genetic factors, whether they are environmental or cultural, has been seen for many diseases.  It’s especially apparent when groups migrate from low- to high-risk countries for a particular condition.  Just this month a study showed that Asian Americans who are more “westernized” have adopted the sunbathing ways of their families’ new homes, which the authors suggest may be the cause of increasing rates of skin cancer in this group.

But the effects of lifestyle changes can also be seen in shifts in disease rates within a population. The prevalence of obesity in United States adults, for example, jumped from 15% in the late 1970’s to nearly 35% today thanks to the trend toward eating more and exercising less.  And because of the increase in obesity, rates of type 2 diabetes are also up.

Many scientists attribute the increase in incidence of several immune system-related disease to what on the surface seems like a good thing about modern lifestyles: fewer infections.  The so-called “hygiene hypothesis” suggests that without the types of infections our species evolved to deal with (many of which are still prevalent in developing nations), our immune systems don’t get the right training.  The lack of challenges to the immune system has been linked to increased rates of allergic diseases like asthma and eczema and autoimmune diseases like Crohn’s and multiple sclerosis.

For some diseases, the reason behind their apparent increases has more to do with increased detection than changes in environment. Up until a few years ago, for example, it was thought that only about one in every 3,000 people in the United States had celiac disease.  But now, thanks to better guidelines on how to diagnose the disease, physicians are finding that about one in every 133 is affected.

On the other hand, some conditions may appear to be increasing because disease awareness is a hammer that makes a lot of people feel like nails.  It has been put forward that restless legs syndrome, for example, is far less prevalent than some estimates suggest and that increases in diagnoses can be traced to “disease mongering” by pharmaceutical companies.

The authors of the Lancet study suggest that the changes in type 1 diabetes rates they are seeing are due to something about modernization.  They point to the fact that the biggest increases were seen in eastern European countries, which have seen the most rapid changes in lifestyle in the last few decades.  But whatever the culprit is, it is obviously not affecting all children.  And that’s where genetic susceptibility comes in.  DNA variations that increase risk may not be changing in prevalence, but type 1 diabetes, like almost every other common disease, is the result of a complex interplay of genes and environment.

(23andMe customers can see how their genes influence their risk of type 1 diabetes in Clinical Reports.)

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Throughout the history of our species there has been one constant:  movement.  Since the origin of Homo sapiens nearly 200,000 years ago in East Africa, humans have journeyed around the globe, ultimately inhabiting every continent save Antarctica.

Scientists have traditionally used archaeology, and more recently genetics, to understand the timing and scope of these ancient migrations.  The field of historical linguistics has also been used in the same way,  reasoning that if people migrated to new regions, they would have brought their languages with them.  In this way, linguistics can be used as an additional resource in understanding our species’ past movements.

However, this concept is rarely – if ever – that straightforward.  For example, a language might spread from local population to local population, while the original speakers of the language stayed put.  This concept, called ‘cultural diffusion’ is at the core of many debates about our species’ prehistory:  did people (and therefore their genes) migrate to a new region, or was it just a transfer of cultural traits (such as language) from one region to the next?

There are two famous examples in human prehistory that delved deep into this very question.

Bantu Expansions in sub-Saharan Africa

About 5,000 years ago, the majority of sub-Saharan African peoples still relied on hunting, gathering, and foraging as their main source of food.  But some people in west-central Africa were developing new techniques of survival.  They began to experiment with herding and agriculture, cultivating the yams, legumes, peppers, and gourds that would became staples of a sub-Saharan African diet.  Then, about 4,000 years ago, they began to move.  As they traveled over a period of centuries, they both displaced and absorbed groups that were already living throughout Africa. 

The languages that they brought with them from their ancestral homeland, belonging to the Bantu family, also spread throughout sub-Saharan Africa.  Today the majority of sub-Saharan African languages are Bantu.

What both genetics and linguistics have told us about the Bantu expansions is that it was an expansion of both peoples and cultures.  Though these early Bantu speakers may have intermarried as they expanded,  their genetic signature still shows West African ancestry.  The distribution of Bantu languages throughout sub-Saharan Africa confirms this prehistoric migration.  In fact, this example marks one of the most straightforward instances of a combined genetic and cultural expansion.  

The Spread of Agriculture and Indo-European Languages

Unfortunately, the spread of agriculture from the Near East into Europe and its connection to the spread of Indo-European languages is far more complicated.  

Agriculture is believed to have originated in the Near East about 13,000 years ago.  Starting about 10,000 years ago, the archaeological record indicates that the practice expanded westward into Europe. By 9,000 years ago, agriculture existed in Greece.  By 5,000 years ago it had reached Scandinavia.

But was this spread of technology accompanied by farmers themselves, as in the Bantu migrations?  Linguists felt they had the answer: the vast majority of languages spoken in Europe today are members of the Indo-European language family.  Languages such as Greek, Latin, Celtic, and English all belong to this group.  In fact, only a few isolated European languages (such as Basque and Finnish) are unrelated.  

Linguists argued that the geographical origin for this language family was probably somewhere in the Near East or Caucasus Mountains, and that these languages spread – along with the farmers themselves – into Europe, replacing the older languages that were already in existence.  The archaeology appeared to support this idea, and soon many scholars had argued for an expansion of agriculturalists from the Near East.

However, the genetics told a somewhat different story.  Though there are genetic footprints in Europe of these early Near Eastern agriculturalists, there are pre-agricultural genetic footprints as well.  In fact, more Europeans trace their ancestry back to ancient European hunter-gatherers, who survived the harsh Ice Age in the southern fringes of Europe 20,000 years ago, expanding into northern Europe as glaciers receded several thousand years later.  They were not replaced or marginalized by the arrival of agriculturalists.

This discordance has made the question of the arrival of agriculture and Indo-European languages in Europe one of the most contentious in the fields of archaeology, genetic anthropology, and linguistics.  It remains unresolved, though many scientists now argue for lower levels of genetic diffusion into Europe than originally thought.

There are countless other examples in human prehistory of cultural vs. genetic diffusion, with different disciplines often yielding different hypotheses.  The connection between genetic diffusion and cultural diffusion is anything but straightforward, and the best research will take into account not only genetic and linguistic evidence, but evidence from the archaeology, human skeletal remains, and paleobotany.  As noted scientist and author Jared Diamond has put it, “It is quite a challenge, but a uniquely fascinating one.”

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