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Similar but not identical: study reveals more about twins than about education

The headlines this week about a new study of genetics told only part of the story

Genes play a bigger role in educational achievement than teachers, schools or home environment, and the reason we know this – apparently – is because we can compare the performance of thousands of pairs of twins.

At least, this was the main conclusion of a “representative” sample of 11,117 identical and non-identical 16-year-old twins, who were used as the basis for the largest research effort in this country into the role that genes and environment play in a range of traits – from the chances of contracting a lethal disease to aspects of personality.

By comparing identical twins, who share identical genes, with non-identical twins, who share half their DNA, scientists are able to tease out the differences that result from genetics from those that come from the environment. This, at least, is the idea. But not all experts agree over the importance of twin studies, and indeed some molecular geneticists are extremely hostile to them. Marcus Pembrey, emeritus professor of paediatric genetics at University College London, for instance, believes they are next to useless when it comes to telling us anything significant about the role of genes.

“In all the years of twin studies I can only think of two occasions when they have produced a meaningful result and I’d be nervous about saying that monozygotic [identical] twins are truly representative of the population. I abandoned my twin studies in 1972,” he said.

Monozygotic twins are one of nature’s idiosyncrasies. A few days after an egg is fertilised by a sperm the developing embryo splits in two, each sharing the same set of parental genes.

Non-identical or dizygotic twins occur when two eggs are fertilised by two sperm and the resulting pair of embryos develops within the same womb, sharing the foetal environment but only 50 per cent of their parental genes, just like ordinary siblings.

The chances of having identical twins does not change much around the world, whereas the probability of non-identical twins can vary quite a bit between countries and over time. For instance, women who became pregnant soon after the end of the Second World War had a higher-than-average chance of carrying non-identical twins. One theory is that the most fertile women fell pregnant first as men came home from the war, and these more fertile women were more likely to ovulate twice during one menstrual cycle.

Although identical twins share the same DNA they are frequently different in many ways. One twin can be larger than the other from birth, indicating an unequal environment in the womb, and it is now firmly established that identical twins can be born with very different health prospects.

John Burn, professor of clinical genetics at Newcastle University, told a London conference on twins this month about the case of a pregnant woman who was an alcoholic. She gave birth to identical twins. One had foetal-alcohol syndrome while the other did not. So even though they shared the same genes and the same foetal environment, the twins were different. “We can’t explain it,” Professor Burn said.

There is also the case of enantiomorphic or “mirror-image” identical twins. Although physically similar, they show certain features that are mirrors of one another – their hair parts on opposite sides of the head or they suck different thumbs when babies, for example.

At the same London conference, organised by the charity Progress Educational Trust, a member of the audience said she and her identical twin sister needed to sit or stand on a particular side of the other in order to feel comfortable. This had been the case for as long as she could remember. “Is this because our mother always put us in our cot on the same side?” she asked.

Professor Burn was unsure, but suggested it might be because she and her sister were enantiomorphic.

It is increasingly clear that identical twins are not in fact identical. This is even more so when epigenetic factors are considered. These control the way genes are expressed and even though the DNA sequences are the same, the way their genes work are almost certainly different.

And yet, the principal assumption behind twin studies is that identical twins share the same genes and, largely, the same environment. This is crucial to working out heritability, which is a measure of how much variation in a particular trait is down to genes.

Heritability is at the centre of the TwinsUK study run by King’s College London, which for 18 years has built up and followed a cohort of identical and non-identical twins. The latest effort on the GCSE performance of 16-year-olds found, for example, that the heritability of compulsory core subjects was 58 per cent, of English 52 per cent, of mathematics 55 per cent and of science 58 per cent.

This is why the scientists concluded that genes played a bigger role in a child’s GCSE performance than any other environmental factor. “We suggest a model of education that recognises the important role of genetics,” the researchers said in their study, published in the journal Plos One.

A serious problem with heritability as a metric for measuring anything, however, is that it varies depending on what you are measuring, on which population it is based, and on the time of assessment. Significantly, few politicians seem to understand this limitation.

The heritability of general intelligence, for instance, rises with age. In infancy, about 20 per cent of a child’s intelligence is attributed to genes, whereas in adults it can be as high as 70 or even 80 per cent, according to Robert Plomin, professor of behavioural genetics at King’s College, who led the twins study into educational achievement.

“Why? Nobody knows, but it’s probably to do with genotype-environment correlation. Genes don’t determine your behaviour. They set up propensities that lead you in one direction or another,” Professor Plomin said.

The importance of this finding for education policy, he said, was that rather than schooling being a passive process of instruction, it should allow active inquiry so that children fulfilled their genetic propensities for learning.

Understandably, Professor Plomin disagrees with molecular geneticists such as Marcus Pembrey who believe that understanding the interactions of individual genes has far greater value than the quantitative approach based on twin studies.

“He’s so wrong I can’t believe it,” Professor Plomin said. “We’ve all heard of the Aesop’s fable about the tortoise and the hare. Well, twin studies have been around for about 100 years and they are the tortoise, while molecular genetics is the hare,” he said. “And we all know who won that race.”