You should spend about 20 minutes on Questions 1–13, which are based on Reading Passage 1 below.
To biomedical researchers all over the world, twins offer a precious opportunity to untangle the influence of genes and the environment – of nature and nurture. Because identical twins come from a single fertilized egg that splits into two, they share virtually the same genetic code. Any differences between them – one twin having younger looking skin, for example – must be due to environmental factors such as less time spent in the sun.
Alternatively, by comparing the experiences of identical twins with those of fraternal twins, who come from separate eggs and share on average half their DNA, researchers can quantify the extent to which our genes affect our lives. If identical twins are more similar to each other with respect to an ailment than fraternal twins are, then vulnerability to the disease must be rooted at least in part in heredity.
These two lines of research – studying the differences between identical twins to pinpoint the influence of environment, and comparing identical twins with fraternal ones to measure inheritance – have been crucial to understanding the interplay of nature and nurture in determining personalities, behaviour and vulnerability to disease.
The idea of using twins to measure heredity dates back to 1875, when the English scientist Francis Galton first suggested the approach and coined the phrase “nature and nurture”. But twin studies took a surprising twist in the 1980s, with studies of identical twins who had been separated at birth and reunited as adults. Over two decades, 137 sets of twins visited Thomas Bouchard’s lab in what became known as the Minnesota Study of Twins Reared Apart. Numerous tests were carried out, and each twin was asked more than 15,000 questions.
Bouchard and his colleagues used this mountain of data to identify how far twins were affected by their genetic makeup. Their key tool was a statistical concept called heritability. In broad terms, the heritability of a trait measures the extent to which differences among members of a population can be explained by differences in their genetics. Wherever Bouchard and other scientists looked, they found the invisible hand of genetic influence helping to shape our lives.
Lately, however, twin studies have helped lead scientists to a radical new conclusion: nature and nurture are not the only elemental forces at work. According to the recent field of epigenetics, a third factor is also in play. In some cases it serves as a bridge between the environment and our genes, and in others it operates on its own to shape who we are.
Epigenetic processes are chemical reactions tied to neither nature nor nurture but representing what researchers have called a “third component”. These reactions influence how our genetic code is expressed: how each gene is strengthened or weakened, even turned on or off, to build our bones, brains and all the other parts of our bodies.
If our DNA is imagined as an immense piano keyboard and our genes as the keys, each key symbolizing a segment of DNA responsible for a particular note or trait, then epigenetic processes determine when and how each key can be struck, changing the tune being played.
One way epigenetics is revolutionizing biology is by revealing a mechanism by which the environment directly impacts genes. Studies of animals have shown that when a rat experiences stress during pregnancy, it can cause epigenetic changes in a fetus that lead to behavioural problems as the rodent grows up. Other epigenetic processes appear to occur randomly, while others are normal, such as those that guide embryonic cells as they become heart, brain or liver cells.
Geneticist Danielle Reed has worked with many twins and thought deeply about what twin studies have taught us. She says it is clear that much of what twins share is hardwired, but also that other things differ. In her view, epigenetics is the origin of many of those differences.
Reed credits Thomas Bouchard’s work for today’s surge in twin studies. She says he was the trailblazer, reminding us that 50 years ago conditions such as heart disease were thought to be caused entirely by lifestyle and schizophrenia was blamed on poor mothering. Twin studies have allowed researchers to be more reflective about what people are born with and what is caused by experience.
Reed adds that the latest work in epigenetics promises to take understanding even further. “Nature writes some things in pencil and some things in pen,” she says. “Things written in pen you can’t change. That’s DNA. But things written in pencil you can. That’s epigenetics. Now that we’re actually able to look at the DNA and see where the pencil writings are, it’s sort of a whole new world.”