HapMap est un catalogue des variations génétiques les plus fréquentes chez l'humain. Il décrit la nature des variantes, leur emplacement dans la séquence d'ADN et leur distribution au sein d'une population et entre les populations dans différentes parties du monde. Le projet international HapMap n'utilise pas l'information recueillie pour établir des corrélations entre des variantes précises et des maladies. Le projet vise plutôt à fournir aux chercheurs de l'information qui leur permettra d'établir des liens entre les variations génétiques et les risques de contracter certaines maladies. Ces recherches pourraient aboutir à de nouvelles méthodes de prévention, de diagnostic et de traitement des maladies.

La construction de HapMap se fait en trois étapes. (a) Les polymorphismes nucléotidiques simples (SNP) sont identifiés dans les échantillons d'ADN provenant de nombreux individus. (b) Les SNP adjacents qui sont hérités simultanément sont regroupés en « haplotypes » dont la fréquence dans la population excède un pour cent. (c) Des SNP marqueurs, au sein d'un haplotype, sont choisis pour constituer un identificateur unique de cet haplotype. En génotypant les trois SNP marqueurs illustrés dans cette figure, les chercheurs peuvent déterminer lequel des quatre haplotypes illustrés est porté par chaque individu. 

http://www.hapmap.org/whatishapmap.html.fr  

How fat genes differ from thin ones

Resequencing effort unpicks genetics of body extremes.

Erika Check

Growth area: resequencing, used here to investigate the genetics of body size, is getting to be big. Alamy

Researchers have used a new technique to hunt for rare genetic quirks that explain why some people are extremely fat or very thin.

The researchers, led by Len Pennacchio of the Lawrence Berkeley National Laboratory in California, studied 757 Canadians from Ottawa. Half of the participants were chosen because they were fatter than 95 of the general population for their height: on average they weighed 125 kilograms. The other half was thinner than 90% of the population, with a mean weight of 57 kilograms.

The team examined 58 genes known to be related to obesity, appetite, or the conversion of food into energy, in every participant. They looked for tiny differences between people in the series of chemical building blocks that make up each gene. This technique, called medical sequencing or resequencing, aims to discover rare genetic variations that may subtly influence particular traits - including body size.

Most previous genetic trawls have focused on using the HapMap - a catalogue of common genetic variants shared by most people with a certain disease. Resequencing is different, as it looks for genetic quirks that are unique to just a few individuals. Resequencing studies have been used to find variations that may cause cancer¹ and differences in cholesterol², whereas HapMap studies have been used to hunt for more common variations that contribute to a range of conditions, including diabetes

http://www.nature.com/news/2007/070312/full/070312-9.html  

Life is faster in the temperate zone

Evolution of species is more leisurely in the tropics.

Michael Hopkin

http://www.nature.com/news/2007/070312/full/070312-8.html

This male 'Masked Tityra' is closer to its sister species than birds in the tropics are to theirs. J. Weir

Most people tend to think of the tropics as the hottest scene on the planet when it comes to spawning new life. But Canadian zoologists have found that it is actually the world's temperate zones where new species evolve and become extinct the fastest.

The discovery by Jason Weir and Dolph Schluter of the University of British Columbia in Vancouver threatens to overturn the theory that because tropical regions contain the greatest overall species diversity, that they must also have the fastest rates of 'speciation' - the emergence of new species.

"Our findings contradict the conventional view by suggesting that temperate zones, and not the tropics, are the hotbeds of speciation," says Weir.

Sisters

The researchers surveyed 309 pairs of 'sister' species - those that are closely related to one another, much like humans and chimpanzees - from throughout the Americas. They compared their DNA sequences to work out how much the sister species had diverged from one another, and therefore how long ago their split had occurred.

Those in temperate zones tended to have diverged more recently, implying that new species are being thrown up faster in these regions. Near the Equator, sister species were separated by an average of 3.4 million years, whereas at the most extreme latitudes studied, stretching as far as the northern wilds of Canada, the figure was less than 1 million years, the researchers report in Science¹.

The apparently prodigious rate at which new species appear and disappear in temperate regions might be due to the cycle of ice ages and warm periods, which affect extreme latitudes more than the tropics, Weir suggests. "Intense climatic instability at high latitudes has resulted in increased opportunities for extinction, and increased ecological opportunity during the benign periods," he says.

By contrast, the relatively unchanged climate of the tropical region over millennia has meant that once species gain a foothold they are less likely to become extinct.

The current warming of the planet from greenhouse gases is also changing climate conditions more at extreme latitudes. But the results of this study apply only to more dramatic changes that happened over a longer time scale; they do not cast any light on how today's species might be affected by future climate change.

Astronomers hash out defense against asteroids

A billion dollars needed to spot potential killer impacts.

Jeff Kanipe

Crash landing: how do we stop this from happening? Don Davis/NASA

Astronomers trying to save the world from Earth-threatening asteroids have this week composed a white paper outlining the threat and what needs to be done about it.

Although it isn't their first white paper on the subject - that was released in 2004 - it is the first mandated by Congress. This, scientists hope, may mean that their conclusions will be taken more seriously by decision-makers in Washington.

In 2005, Congress passed a bill authorizing NASA to search for asteroids as small as 140 metres that could possibly strike the Earth. The bill, however, provided no money for the search. Scientists at the Planetary Defense Conference, held 5-8 March in Washington DC to compose the new white paper, were quick to point this out.

On Monday, Simon 'Pete' Worden, director of NASA's Ames Research Center, said that the cost of finding at least 90% of the 20,000 estimated potential Earth-killers by 2020 would cost about $1 billion. US government employees, including NASA scientists, don't usually make public requests for more cash, but Worden was clear: "We know what to do, we just don't have the money."

Earth's magnetic field reversals mimicked in the lab

The switching of the poles can be studied in a tub of molten metal.

Philip Ball

Poles apart: our planet's field flips every now and then - though no one knows why. NASA

Every few hundred thousand years or so, the Earth's north and south magnetic poles switch places. No one knows what triggers these geomagnetic field reversals, but a team in France has now reproduced them in the lab.

Michaël Berhanu of the École Normale Supérieure in Paris and his co-workers have spun enough molten sodium to fill a small bathtub in a copper cylinder at many revolutions per second. This provided a rough simulation of the Earth's spinning core of molten iron¹.

Electrical currents that are produced spontaneously in these swirling liquid metals set up a magnetic field in a process known as dynamo action. The same principles are used in industrial dynamos to create electricity, by moving metal wires through a magnetic field. Although it has been known for some time that turbulent motions in the Earth's core create the geomagnetic field this way, the details of how that happens still aren't clear.

In particular, geomagnetic reversals remain puzzling. Our magnetic poles have, in the past, faded away and then re-emerged upside down. These events are recorded in magnetic sedimentary rocks, which reveal the strength and direction of the prevailing field when it was formed. But no one knows why the reversals occurred when they did.

http://www.nature.com/news/2007/070305/full/070305-14.html