Jerome Groopman writes:

As I am writing this, my DNA is changing. And, as you read this, so is yours. People tend to assume that the genes we inherit from our parents are a fixed blueprint for our growth and development, immutable throughout our lives, and that the DNA in each cell of our body is the same as in every other cell. In fact, changes in our DNA, known as mutations, occur from the time we are in the womb until our death—a phenomenon that has become increasingly important in medical science as our understanding of human genomics becomes more sophisticated. In “Beyond Inheritance” (Riverhead), the science journalist Roxanne Khamsi provides a useful guide to this body of research and its far-reaching, sometimes surprising implications. “You are a slightly different genetic version of yourself today from yesterday, and will be different yet again tomorrow,” she writes.

These kinds of mutations are called somatic mutations, derived from the Greek word soma, which means “body,” and they are less familiar to most people than inherited mutations are. A famous example of the latter is the hemophilia that affected many of the male descendants of Queen Victoria. Victoria, in common with some of her daughters and granddaughters, was a carrier of a mutation for hemophilia. None of these women manifested the traits of the disease, because the mutation occurred on one of their two X chromosomes, the other of which lacked the mutation. But three generations of male offspring, each having only one X chromosome, inherited the full-blown bleeding disorder.

Although hemophilia is usually a hereditary condition, there are some patients whose relatives do not have the abnormal gene. These people have a somatic mutation. Few of us are unfortunate enough to spontaneously develop such a severe malady, but somatic mutations occur in everyone all the time; we are all collections of errors, mosaics of altered DNA. Each time a cell divides, its DNA is copied, but mistakes inevitably creep in. The adult body, after all, contains some thirty trillion cells, about four million of which are replaced in any given second, and the human genome is made up of six billion letters of DNA. “By some estimates you acquire trillions of new mutations a day,” Khamsi writes. Environmental factors—such as radiation, sunburn, air pollution, and smoking—can increase the rate at which errors in DNA copying happen, but genetic mutations are not rare aberrations; they are intrinsic to the system by which cells reproduce. [Continue reading…]