Saturday, December 14, 2013

Same but not the same

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. 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. 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. 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.

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.

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. It is increasingly clear that identical twins are not in fact identical.

“Genes don’t determine your behaviour. They set up propensities that lead you in one direction or another,” Professor Plomin, professor of behavioural genetics at King’s College said.

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