Identical twins don't always have identical DNA, according to a new study.
Identical twins differ by 5.2 genetic changes on average, according to a new January 7 report in Nature Genetics, as reported by Science News. This study goes contrary to the belief that identical twins are identical in genetics, which was often the basis for the idea that differences in health between twins is a result of their environment. This new study shows that genetics might have more to do with that than previously believed.
"Imagine you have identical twins who are reared apart," DeCode Genetics CEO and founder, and study co-author, Kari Stefansson, told CNN. "If one of them developed autism, the classic interpretation is that it is down to environmental factors. But our work shows that before you conclude that it's caused by the environment, you have to sequence the genome of the twins to know what could account for the autism."
Stefansson said this "mutation divergence" could be the cause of "devastating childhood illnesses" such as epilepsy and metabolic disorders. She also said a large percentage of "such horrific syndromes of very early childhood are down to genome mutations."
Identical twins, which are also known as monozygotic twins because they come from a single fertilized egg as noted by Science News, are often studied to determine the origin of specific traits, diseases, or conditions, be it the result of genetics or the result of their environment. This study in particular looked at 381 pairs of identical twins and determined that 38 of the pairs had identical genetics, but that most pairs had some differences in DNA that arose very early in their development.
Thirty nine of the pairs had over 100 changes between the twins and the study suggests that these DNA differences occur when the embryo splits to form two in early development or shortly after the embryo splits. It suggests that this embryo split isn't as clean or neat as previously thought, which results in the DNA changes seen.
Some twins arise when a single cell or small group of cells split off of the embryo, Science News notes, and it's the number of cells that a twin originates from that can determine the genetic differences between it and its other twin. The more uneven splits of the embryo that occur, the larger the number of genetic differences between twins.
"This is an extraordinary, exciting, and insightful effort to pinpoint early cellular mechanisms that explain genetic differences between MZ (monozygotic) twins," California State University, Fullerton, psychology professor, Nancy Segal, told CNN. "It is well known that MZ co-twins do not show perfect resemblance and that some dissimilarities may reflect genetic differences. The present study offers new information as to the source of the MZ co-twin differences."
Segal said the study shows that "some twin models underestimate genetic effects and require revision."
For more twins, check out our list of the best movie twins ever, and if you're looking for more DNA science, read this story about how a new study reveals that most Vikings weren't blond or blue-eyed.
Wesley LeBlanc is a freelance news writer and guide maker for IGN. You can follow him on Twitter @LeBlancWes.
