Our Twisted DNA | by Tim Flannery | The New York Review of Books
As long as chimeras and mosaics were detected on the basis of physical manifestations or blood type, they were considered to be phenomenally rare—indeed freakish. By 1983, only seventy-five cases of human chimeras, as detected from blood type, were known, while mosaicism was mostly known from medical cases. Joseph Merrick, the “Elephant Man,” suffered from a form of mosaicism known as Proteus syndrome, which left parts of his body deformed by monstrous growths, while other parts remained completely normal. For decades, his sad example defined the condition for many.
Recent advances in genetic analysis have revealed that chimerism is common. In fact, chimeric individuals may be the rule, rather than the exception, among mammals. One Danish study of the blood of 154 girls aged ten to fifteen discovered that around 13 percent of them had blood cells with Y-chromosomes. These cells probably originated from an older brother and had crossed into the mother, where they survived before crossing into, and taking root in, the daughter. A Seattle study of fifty-nine women who died, on average, in their seventies found that 63 percent had cells with Y-chromosomes in their brains.
As bizarre as chimeras might seem, they represent only the surface waters of Zimmer’s deep dive into the nature of inheritance. Epigenetics, a fast-expanding area of science that explains how things experienced by individuals can influence the traits that are inherited by their offspring, seems to contradict our conventional understanding of genetics. The epigenome, “that collection of molecules that envelops our genes and controls what they do,” as Zimmer puts it, operates through methylation—the process whereby methyl-group molecules are added to the molecular envelope surrounding the DNA, and so inhibit certain genes from operating (and, in some cases, from operating in descendants as well).
We owe one of the most penetrating insights into epigenetics to a laboratory accident. Michael Skinner of Washington State University was examining the impact of the anti-fungal agent vinclozolin on laboratory rats. He discovered that the offspring of rats exposed to the chemical produced deformed sperm. When a laboratory assistant accidentally used these offspring to breed a new generation of lab rats, researchers discovered that the grandsons of the poisoned rats also produced deformed sperm.
Skinner’s rats sparked a flurry of new experiments that showed how methylation could lead to the inheritance of acquired traits. As some researchers commented, it was as if the work of Jean-Baptiste Lamarck (who famously posited that the necks of giraffes had lengthened over generations because they were stretched as the animals reached up to feed) had become reestablished. Science is rarely so simple—still, epigenetics has Zimmer wondering whether “poverty, abuse, and other assaults on parents also impress themselves epigenetically on their children.” The study of epigenetics is still in its infancy, so it may be years before we know the answer. With some recent studies showing that epigenetic effects fade over time, many researchers are unsure whether epigenetics is anything but an interesting codicil to the conventional genetic theory of inheritance.