This week, a mom posted a question on my Facebook page: “If twins are identical why do they have different issues? I thought they share the same genes.”
Yup, monozygotic (MZ) or identical twins do share the same genes. 100 percent of their DNA, in fact. Whereas dizygotic (DZ) or fraternal twins are merely two siblings born on the same day and share only 50 percent of their genetic make up at best, identical twins are genetic clones of one another. That’s why identical twins look and sound remarkably alike, score within points of each other on IQ tests, and even have similar likes and dislikes.
Still, if you’ve ever met identical twins or have them in your own family, you’d know that that’s not always the case and in fact, many identical twins are very different. From subtle nuances in their personalities to taste in food and music, identical twins often show preferences that are markedly different from their cotwins.
Many of these variations in character can be explained by outside influences or the environment (the “nurture” side of nature-versus-nurture theory). Peers have a strong influence on all kids including twins and often set the tone from what’s cool to wear to what sports to play. Plus, once they reach the school years many twins have a need to be seen as “different” from their cotwins and will go to great lengths to show the world that they are indeed an individual. (French psychologist Rene Zazzo called this twin phenomenon “The Couple Effect.”)
But that explains personality differences. What about genetic differences? Yes, I know that I just wrote that identical twins have the exact same DNA, and they do, but believe it or not, this identical DNA acts differently. How do we know? Well, some identical twins are not susceptible to the same diseases as their cotwins. For instance, if one twin is diagnosed with Hodgkin’s disease, a highly inheritable cancer, it’s actually very rare for both identical twins to develop it. Furthermore, identical twins have a very low rate of concordance (presence of the same trait) for many psychiatric diseases such as schizophrenia and bipolar disorder. And what about developmental disorders like autism and even attention deficit disorder (ADD)? Again, while the rate of concordance is higher for identical twins than it is for fraternal twins, it’s by no means a slam dunk–just because one identical twin is diagnosed with the disorder, the other may never develop it. By why is that?
Scientists believe that although twins have the same genome (or DNA), they have different epigenome (the way that their DNA is “organized” in a cell). Throughout their lives, something changes or modifies the epigenome causing one twin’s genes to react differently than her cotwin’s. Although researchers aren’t quite sure what triggers the changes to the epigenome, many speculate it has something to do with the environment. And research has shown that while the epigenome of very young twins is nearly identical, the older they get, the greater their epigenome differs! As they grow up, identical twins move on with their own lives and come into contact with different people and places. They attend different colleges, start their own careers, get married, move to different parts of the country and so forth. As a result, they come into contact with vastly different environmental factors. And this, it seems, affects their epigenome.
So the next time you hear someone say, “Wow, they’re just so identical!” You can just smile knowing that that’s rarely the case!