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You've probably seen the splashy headlines declaring that astronaut Scott Kelly and his brother are no longer identical twins (as originally claimed in a since heavily corrected article by LiveScience). Media outlets reported that 7 percent of astronaut Scott's DNA had changed as a result of him spending 17 months in space. These claims stem from a NASA report of preliminary findings (rather than a full research report, which means that details at this point are thin). What we do know, however, is that these attention-grabbing statements arose from a fundamental miscommunication about the biology of life: It's not 7 percent of his genes that changed, but 7 percent of the expression of those same underlying genes. As attempts to correct this misunderstanding have sprung up, it's important to understand what actually happened, and what we can learn from the Kelly twins' DNA.
Sequence DNA Versus DNA Expression
When we're talking about DNA or genes, we're almost always thinking about the sequence of DNA — that is, the arrangement of proteins (available in four types, labeled with the letters A, C, T, and G) that make up the rungs of the ladder of the physical DNA strand. Although mutations, or changes, to this underlying sequence do occur, they are relatively small (affecting very small sections of the DNA sequence), and — when they occur after the chromosomes have merged between the sperm and the egg — tissue-specific (not affecting the entire organism).
Another way to think about the effect as it was originally reported is to consider the genetic similarity between humans and non-humans. All humans have >99 percent of the exact same DNA; it's what makes us human. We also share around 95 percent of our DNA with chimpanzees (our closest non-human cousins) and 85 percent of our DNA with a zebra fish. (Why? Because the majority of protein-coding DNA is working at the basic tasks of life — respiration, nutrition, reproduction, etc.) If 7 percent of Scott's DNA sequence had changed during the year he spent in space, he wouldn't just not be his brother's twin anymore; he wouldn't even be human. (He might be a rhesus monkey; we share 93 percent of our DNA with them.)
What did change was how 7 percent of his genes were expressed. Genetic expression is broadly the process by and extent to which the underlying DNA sequence is "read," or how it creates proteins. Gene expression is how different tissues are created from the same DNA. Cells in your brain have the same DNA sequence as cells in your kidneys; those cells perform different functions, because of differences in how the same DNA sequence is expressed. Changes in gene expression can also cause changes in your body's biology over time. "Puberty genes" are always present in the DNA sequence, but they're only expressed (or expressed differently) when your body goes through that awkward phase of adolescence.
What Can We Learn From Astronaut Scott and His Still-Twin Brother?
Scientists already knew that exposure to different environments has the potential to massively alter gene expression. Smoking causes widespread changes in gene expression (or, at least, biological markers of it). This is actually something that makes large-scale research into human gene expression very difficult; we know that if we're interested in the impacts of other environments or behaviors, we have to be careful that we're not comparing smokers to non-smokers as well. Even if astronaut Scott and his twin had both spent that year on Earth, there would be differences in their gene expression profiles, because they would have been exposed to different environments, even if much less dramatically. In fact, we don't have enough information to know if the 7 percent difference in gene expression is larger than what would have been observed had they both been on Earth. We do know that gene expression becomes more and more different between identical twins as time goes on. It could be that the experience of being weightless in space changed astronaut Scott's gene expression, but some of the observed differences could have been due to the different foods that he and his brother ate, or different levels of stress they experienced — differences that can occur on Earth, too.
What we learn from the tale of Scott and his twin is the same reason behavior geneticists have long had a particular fascination with twins: Identical twins are an example of a natural experiment. They are two individuals who happen to be born with the same DNA at the same time. Despite their incredible similarity at the genetic level, identical twins are certainly not the same person. And that's what makes identical twins fascinating to scientists — how is it that two people, born at the same time with the same set of genes, more-often-than-not raised in the same family, can turn out so differently? The answer is, of course, the influence of the environment that each twin experiences uniquely from the other. And yet, on the flip side, identical twins are far more similar to one another, physically and behaviorally, than any other possible pair of two people (even siblings or fraternal twins, who are essentially siblings who happen to be born at the same time). This means that genes (or the only thing that makes identical twins more similar to one another than any other pair of two people) must also play a role. This is the fundamental conclusion of behavior genetics: It's not a matter of nature versus nurture, but rather nature and nurture.
