It is now regarded as a simple fact of life that you are stuck with the genes your parents gave you. Genetic mutations from long ago have passed down from generation to generation, whether it is joined earlobes, knobbly knees or a higher risk of breast cancer. This is Darwinian inheritance in action.
But what if that isn’t the full story? What if your actions as a ten-year-old not only affected your genes, but those of your future children and grandchildren?
Darwin wasn’t the first to offer a theory on evolution; before him was Jean-Baptiste Lamarck, who suggested that traits developed by animals during their lifetime could be passed onto their children. The classic example is a giraffe, who, after years of stretching to reach higher leaves, would have offspring with longer necks. After several generations of stretching, giraffes would have necks as long as they are today.
This made intuitive sense and even Darwin was reluctant to dismiss it — he was not quite the Darwinist he is now made out to be. However, now that DNA is known to be responsible for passing on genetic information, it is hard to see how this could work. No amount of neck-straining will change your genes, so surely this cannot happen.
The Human Genome Project aimed to transcribe our DNA and find out what makes a human. Even after decoding all three billion ‘letters’, there was still not enough information to provide all the answers. One of the problems is that many genes can be switched off, or ‘silenced’, by the body. This occurs through methylation, where a small marker, just one carbon and three hydrogen atoms, is attached to a section of DNA and switches it off. Since this silences a gene without actually altering the genetic code, methylation is described as epigenetic, from the Greek ‘epi-’ for ‘above’. This system of silencing genes is another layer of information on top of the genetic code — an epigenetic code.
Methylation of DNA is a natural way that an organism regulates itself, but external factors can affect this dramatically. When a honey bee larva is fed on royal jelly its methylation levels fall, causing it to develop ovaries, grow larger and live longer, i.e. become a queen. However, it is not just food that is important; mice change their DNA methylation patterns depending on how much attention their mothers paid them as a pup.
Epigenetic changes due to environment can happen even before birth. Studies of several famines, including the Dutch Hunger Winter of 1944 and the Great Chinese Famine of 1958–61, have shown that children conceived during these periods were underweight at birth. Now adults, they have increased rates of obesity, heart disease and schizophrenia, and have increased methylation of genes linked to these diseases. This makes clear the importance of epigenetics as not just an aside to genetics but a powerful force in its own right.
It was long thought that epigenetic changes could not be passed to our children, as sperm and egg cells undergo ‘reprogramming’ to wipe the slate clean. However, recent experiments have shown that mice are able to pass on epigenetic changes and historical records suggest that this may be happening in humans too.
As an isolated farming community in the early 1800s, Överkalix in north Sweden suffered from extreme fluctuation in food supply; some years would be plentiful while others would be ruinous. This variation encouraged Lars Olev Bygren, a preventative health specialist, to trace the ancestries of a hundred villagers and cross-reference them with harvest records. Remarkably, he found that boys who had survived a year of famine while aged 9–12 went on to have sons and grandsons who lived on average 32 years longer than those of boys who had enjoyed a year of feasting. This prepubescent stage is when the body is most susceptible to environmental changes; however it is remarkable that the repercussions can be seen two hundred years later in the lives of their grandchildren, who were never directly exposed to famine.
This seems very similar to Lamarck’s theory that the impact of an animal’s life is passed down to the next generation. However, epigenetics cannot replace Darwinian inheritance, where a genetic mutation is permanent, as epigenetic changes should only be temporary. But how long is temporary? We have seen effects passed down at least two generations in humans, while experiments with roundworms have shown epigenetic changes surviving over 40 generations.
Epigenetics gives us the flexibility that genetics could never provide, allowing us to adapt to our environment during our lifetime. However, it is not just our own lives being adjusted; we could be determining the lives of our future children. Take heed, the sins of the father may well be visited upon the son.