IDENTICAL TWINS—IT’S NOT JUST DNA.

IDENTICAL TWINS—IT’S NOT JUST DNA. I am an identical twin so I am aware of the physical differences in identical twins as well as the striking similarities. Some of the differences between my brother and me seem explainable by environmental factors. Some, however, seem as if they ought to be governed by genetic factors—and so, it seems, we should not differ. An example is allergies. My brother Elmer has had a severe reaction to pollen since he was in his early twenties. There are similar allergies on both sides of our family. I have only had mild reactions on rare occasions to heavy exposure to pollens. This article in the Economist for January 24 points out that there is a secondary “epigenetic” inheritance mechanism parallel to the primary DNA-based mechanism. I don’t understand the explanation, but the result is that the behavior of genes—and not just the genes themselves— is important in determining characteristics of organisms. The behavior of genes is affected by a process called “methylation” and the pattern of methylation is also passed on from parent to child. A recent study of methylation patterns in twins shows that there is enough variation in the methylation patterns of identical twins to account for some important physical differences, including the inheritance of a disease by one identical twin, but not the other. Yet the methylation patterns of identical twins were found to be more similar than those of fraternal twins—so that epigenesis operates to make identical twins more similar than other siblings.