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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Melissa Block.

Scientists today are reporting a new explanation for why identical twins aren't exactly identical. They already knew that external forces, like diet and exercise, could explain some of the differences, but scientists suspected something else was going on. Now, they think they know what it is.

And as NPR's Joe Palca reports, the answer comes from worms.

JOE PALCA: Before we get to the really cool stuff involving worms, let's talk a little bit about people.

Dr. ARJUN RAJ (Molecular Biologist, University of Pennsylvania): If you look in identical twins, how different are identical twins?

PALCA: That's Arjun Raj. He's a molecular biologist at the University of Pennsylvania. He says the differences aren't great, but since they have the same genes there must be something else going on.

To systematically study that something else, you'd want to rear twins in identical circumstances for the first 20 or so years of their lives - same diet, same time on the treadmill - so you can eliminate the external forces that might be causing the differences.

Well, you know as well as Arjun Raj does that raising humans in captivity is frowned upon, so he decided to study genetically identical worms.

Dr. RAJ: Think of them like identical twins.

PALCA: He reared his worms in the same petri dishes with the same food, yet they didn't all grow up the same.

Dr. RAJ: Some fraction of them will look one way and some fraction of them will look differently, even though they're really identical.

PALCA: So, what's going on here? Raj thinks the answer is how genes express themselves. When scientists talk about gene expression, they mean is the gene turned on? Raj figured out a neat way to watch genes turn on and off. When a gene expresses itself, it makes a molecule called messenger RNA in various spots in the developing worm.

Dr. RAJ: We have a really nice method for actually counting individual molecules by fluorescence imaging, so you can actually see these individual little spots of messenger RNAs. And by counting those spots, we get a really accurate estimate of how much expression there is.

PALCA: So, he studied one particular gene and measured how it was expressing itself as worms grew from embryo to adult. And even though the DNA in these worms was all identical, the gene expression was not.

Dr. RAJ: We think that this variability is stochastic.

PALCA: Stochastic - big word, but all it means is the variability is random; no specific reason for it. Just random.

In this particular case, the results of this variation is some worms develop normally and some didn't. So, coming back to boring old humans, some of the reason identical twins aren't exactly alike may be because of these random fluctuations in gene expression.

But what a minute. If all these genes are expressing themselves differently as we grow from embryo to adult, how come most of the time we develop normally? University of California San Diego Biologist Scott Rifkin says that's a good question.

Dr. SCOTT RIFKIN (Biologist, University of California-San Diego): You have all these things that can go wrong. You have all this genetic variation. You have all these fluctuating environments, and yet it works. You know, you don't see people going out there with third arms growing out of their chests.

PALCA: Rifkin was Raj's co-author in the worm study that is published in today's edition of the journal Nature.

Now, there's one thing I haven't told you about the worm experiment: These were mutant worms engineered to reveal the gene expression fluctuations.

Adrian Streit is a biologist at the Max Planck Institute in Tubingen, Germany. He says you don't see this variability most of the time because there's a lot of redundancies in the system - multiple genetic pathways that can get the same result.

Dr. ADRIAN STREIT (Biologist, Max Planck Institute): Basically, by having two or three things that do the same, even if one of them might not be right 100 percent of the time, if you have two or three that do it, it's very, very unlikely that all of them fail.

PALCA: And that built-in redundancy comes to us courtesy of evolution. Scott Rifkin says natural selection weeded out the creatures that didn't work out reliable systems to make sure arms and legs and kidneys all show up in the right places.

Dr. RIFKIN: The ones that didn't work, well, they didn't pass on their genes. They didn't pass on the whole developmental process to their offsprings. They didn't have any offspring. They didn't work.

PALCA: So, celebrate our differences but be glad evolution gave us backup systems so we mostly look the same.

Joe Palca, NPR News, Washington.

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