You inherit some of your grandmother's genes, but do you also inherit her experiences? Last month, a group of Swedish researchers at the Karolinska Institute and the University of Umeå released their latest in a series of reports on a long-term health study of people born between 1890 and 1920 in Överkalix, a small town in northern Sweden. These scientists have been making waves for more than a decade with claims that our health is influenced by the experiences of our grandparents. Using statistics of 19th-century harvests in Northern Sweden to determine how much food was available to the ancestors of the residents of Överkalix, the researchers concluded that the risk for cardiovascular disease among their study participants was influenced by the dramatic swings from feast to famine experienced by the participants’ grandparents during childhood.
Studies like this one are part of the hodgepodge of research that gets lumped together in the growing and increasingly ill-defined field of epigenetics. Scientific interest in this field has boomed over the past decade, thanks in part to technological advances that make new types of experiments possible. Epigenetics is hot in the popular press as well. It made the cover of Time in 2010. In Germany, Der Spiegel declared that epigenetics is a "victory over the gene," illustrating both the victory itself and the sexiness of the science with an image of naked woman emerging from the confines of her gene pool. And of course, epigenetics is touted as the new secret to curing cancer. But the popularity of epigenetics is misplaced: It's a badly over-hyped field whose recent findings aren't nearly as revolutionary as many of its practitioners believe.
The epigenetic states of the DNA in your ancestors' liver, muscle, and neurons are irrelevant. The real mystery is how the epigenetic effects of some environments and life experiences manage to get transmitted to our reproductive systems.
What is epigenetics? The term was coined in 1942 by British biologist Conrad Waddington, as a name for the study of how genes produce a fully developed organism from a single fertilized egg. More recently, epigenetics is commonly defined as the study of processes that transcend our genes—processes that produce different outcomes from a single, fixed set of genes by controlling how those genes get used. A classic example is a strain of laboratory mice that carry a particular version of a gene that can cause two different coat colors. Two mice may carry identical versions of this gene but nevertheless appear very different, with either light or dark fur, depending on its epigenetic state—whether the gene is in an on or off state.
What is surprising about this gene is that its epigenetic state can be passed from parents to offspring. This is not supposed to happen. Biological development depends on a complex process of switching the epigenetic states of tens of thousands of genes, as a newly fertilized egg gradually transforms itself into an adult organism made up of trillions of cells that come in hundreds of different types. In order for all of this epigenetic switching to happen correctly, genes must reset to a default state at conception. But researchers are finding evidence of exceptions, genes that somehow escape the reset. If states of genes can be inherited, and not just the genes themselves, then life experiences that happen to alter the epigenetic state of your grandparents' genes can be passed on to you.
Recently developed technologies make it possible for scientists to measure the epigenetic states of genes more comprehensively than ever before. The result is a wave of studies showing how our life experiences impact the state of our genes. These studies are fascinating, but the results shouldn't be particularly surprising to anyone familiar with the last 60 years of molecular biology. DNA is not a static biological blueprint. Much as your body adapts to a high-altitude environment by ramping up the production of red blood cells, our genomes respond to environmental signals by changing which genes get expressed. This is not news. A trio of French scientists won a Nobel Prize in 1965 for showing how this process works in bacteria, and there have been decades of studies of this phenomenon in humans.
So we shouldn't be surprised that our physical and even our mental environment can influence the behavior of our genes in many different cells in our body. Epigenetic states of genes are manifestations of DNA doing its job, and many scientists are currently focused on trying to understand what those different epigenetic states mean. This research is important and interesting, but not paradigm-breaking.
The question of inherited epigenetic states is another matter. When it comes to inheriting your grandmother's experiences, only a few epigenetic states count: the states of the genes in reproductive cells. DNA get passed on only through sperm and egg cells, and so the epigenetic states of the DNA in your ancestors' liver, muscle, and neurons are irrelevant. The real mystery is how the epigenetic effects of some environments and life experiences manage to get transmitted to our reproductive systems.
Only a minority of epigenetic research is focused on this specific issue, and it's unclear how widespread the phenomenon actually is. In the case of most human studies, as one review noted, "cultural confounders are almost impossible to rule out." The University of Bristol's George Davey Smith, after reviewing the evidence, wrote that "The conclusion from over 100 years of research must be that epigenetic inheritance is not a major contributor to phenotypic resemblance across generations."
Despite such justified skepticism, the epigenetics field on occasion produces a provocative result that is hard to dismiss. In January, Brian Dias and Kerry Ressler of Emory University reported that male mice conditioned to be startled by a particular odor passed on that startle response to their offspring. This effect persisted for two generations, despite the fact that the offspring were conceived by in vitro fertilization and never had any physical or social contact with the original, odor-conditioned mice. As far as I can tell, this study was properly done, and includes all of the right controls to rule out confounding effects. Nobody has any idea how a specific, learned behavior gets epigenetically transmitted to sperm, preserved through embryonic development, and embedded in an adult offspring. Epigenetics, like many developing scientific fields when they enter the limelight, is rife with overblown claims and weak studies. But it's a field to watch—despite the hype, there are genuine mysteries to solve.