Twins: Nature, nurture, and epigenetics

National Geographic has a really interesting story on what we can learn about human biology and human culture from studying the lives of twins. (Last week, Mark blogged about some of the photos in the story.) The story explains the chance beginnings of the now-massive Minnesota Study of Twins Reared Apart; introduces you to twin girls from China who were adopted by two different Canadian families that now work to keep the girls in each other's lives; and delves into what we know and don't know about why some identical twins are different from each other in very conspicuous ways.

One example of this last bit is the story of Sam and John, identical twin brothers. Both are on the autism spectrum, but they appear to be on entirely different parts of that spectrum, with John experiencing much more severe symptoms that led the boy's parents to enroll him in a special school. Why would identical twins, raised in the same family, have such an obvious difference in the expression of characteristics that are probably mostly inherited? That's where epigenetics comes in.

A study of twins in California last year suggested that experiences in the womb and first year of life can have a major impact. John's parents wonder if that was the case with him. Born with a congenital heart defect, he underwent surgery at three and a half months, then was given powerful drugs to battle an infection. "For the first six months, John's environment was radically different than Sam's," his father says.

Shortly after Sam and John were diagnosed, their parents enrolled them in a study at the Kennedy Krieger Institute in Baltimore. Blood samples from the boys were shared with a team at nearby Johns Hopkins University looking into the connection between autism and epigenetic processes—chemical reactions tied to neither nature nor nurture but representing what researchers have called a "third component." These reactions influence how our genetic code is expressed: how each gene is strengthened or weakened, even turned on or off, to build our bones, brains, and all the other parts of our bodies.

If you think of our DNA as an immense piano keyboard and our genes as keys—each key symbolizing a segment of DNA responsible for a particular note, or trait, and all the keys combining to make us who we are—then epigenetic processes determine when and how each key can be struck, changing the tune being played.

Image: Twins, a Creative Commons Attribution (2.0) image from missbossy's photostream

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