Despite being separated at birth, identical twins were astonished at how alike they were in looks and manner, but just how identical are identical twins?, asks Claire O'Connell
LAST WEEK it emerged that identical twins who had been accidentally separated at birth 35 years ago are now suing the Spanish health authorities for the impact it has had on their lives.
In a mix-up at the hospital in the Canary Islands, the parents brought home two babies, but only one was theirs by birth. The other twin had been swapped by mistake with another couple's baby.
The families only became aware of the blunder by chance, when a friend of one identical twin served the other in a shop. When the shop assistant introduced the two "real" twins, they were astonished at how alike they were in looks and manner, and one reported that it was "like looking in a mirror".
But despite the surface similarity, it's a fair assumption that the twins would by then have had differences at a genetic level, according to development expert Prof Richard Tremblay, who says that as identical twins age, they display genetic differences that can have an impact on their health.
So just how identical are identical twins? Identical, or monozygotic, twins develop from the same fertilised egg, so they start off with essentially the same DNA.
But differences in their environments will have an "epigenetic" effect on when individual genes are turned on or off, explains Tremblay, who last year moved from Canada to take up the professorship in child development at University College Dublin.
"In principal is completely identical at the start," he says. "But then there are epigenetic changes. So although the genes are identical, some genes are turned on in one twin and not in the other."
Even in the womb twins can experience important differences in their environments, notes Tremblay.
"One can have less space than the other and the blood they receive is different. So, even though they share the same environment, there are important differences in that environment and it can create epigenetic differences."
This prenatal phase is a particularly important window for development, and involves an orchestra of genes switching on and off, explains Tremblay.
"Genes are coding for the development of our bodies, and the turning on and off defines that your nose is where it is," he says.
"Most of the development, around 80 per cent, will have happened by the time we are born. But the 20 per cent that remains postnatally is extremely important too."
One of the signatures we carry from our time in the womb is our fingerprints, which form through a mix of genetics and environment, and differ even between identical twins.
"The fingerprints are the result of what happens during early pregnancy," says Tremblay.
"They are like scars that are left over from what happened during development. You can see that if you have a twin that had less space and less blood, his environment is poorer than the other twin although they had the same genes at the start, and the environment will have an impact at that level.
"And it's the same thing for the brain, or any other organ."
Twin studies have shown that as a pair of identical twins ages, their genetic expression can diverge further.
"It's clear that there are more and more differences with age," says Tremblay. "If we look at gene expression we would see important differences and that will have an impact on physical and mental health.
"And the environment has an impact on gene expression throughout life, so you can get the twin who has cancer and the other one doesn't have cancer," he says.
Twin studies provide a valuable model for scientists to tease out the relative effects of nature and nurture, explains Tremblay.
"It's one of the big areas of research at the moment, taking monozygotic twins and looking at the differences between them. It's the best way to look at the effects of the environment," says Tremblay.
He is currently involved in a 12-year study of 500 pairs of twin children (identical and non-identical) in Québec, looking at influences of genes and the environment on a range of factors, including obesity, autism, stress response and aggression.
One aspect of the study found that a tendency towards physical aggression is chalked up to genes until toddlerhood, and then the environment takes over. "We showed that physical aggression at 17 months of age was highly genetically determined, and over time it's more environmentally determined, that the environment has a strong impact on development of physical aggression," explains Tremblay.
He is also working with Perinatal Ireland, a research consortium of maternity centres here, on a prenatal study of twins, which uses ultrasound to monitor the babies as they develop in the womb.
One of the aims is to look at gene expression in twins at birth using samples of umbilical cord blood, says Tremblay. "It's amazing but no one has done that yet, we are hoping to be the first."
And he hopes the prenatal and birth data on twins will help piece together the puzzle of how genes and environment work together to impact on development.
"The important thing is that we will be able to link the prenatal development to the genetic expression as we see it at birth, and then understand it over time as we follow it," he says.