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Research using twins
A
To researchers all over the world, twins offer a opportunity to the influence of genes and the environment - of nature and .
Because come from a single that splits into two, they share the same code.
Any differences between them - one twin having younger looking skin, for example - must be due to such as less time spent in the sun.
B
, by comparing the experiences of identical twins with those of , who come from separate eggs and share half their DNA,researchers can the extent to which our genes affect our lives.
If identical twins are more similar to each other an than fraternal twins are, then to the disease must be rooted at least in part in .
C
These two lines of research - studying the differences between identical twins to the influence of environment, and comparing identical twins with fraternal ones to measure the role of - have been crucial to understanding the of nature and nurture in determining our personalities, behavior, and vulnerability to disease.
D
The idea of using twins to measure the influence of 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 in the 1980s, with the arrival of studies into identical twins who had been separated at birth and as adults.
Over two decades 137 sets of twins eventually visited Thomas Bouchard's lab in what became known as the Minnesota Study of Twins Reared Apart.
tests were carried out on the twins, and they were each asked more than 15,000 questions.
E
Bouchard and his colleagues used this mountain of data to how far twins were affected by their .
The key to their approach was a concept called heritability.
In broad terms, the heritability of a measures the to which differences among members of a can be explained by differences in their genetics.
And wherever Bouchard and other scientists looked, it seemed, they found the hand of genetic influence helping to shape our lives.
F
Lately, however, twin studies have helped lead scientists to a new conclusion: that nature and nurture are not the only forces at work.
According to a recent field called epigenetics, there is a third factor also , one that in some cases a bridge between the environment and our genes, and in others operates on its own to shape who we are.
G
Epigenetic processes are 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 or weakened, even turned on or off, to build our bones, brains and all the other parts of our bodies.
H
If you think of our DNA as an piano keyboard and our genes as the keys - each key symbolizing a segment of DNA responsible for a particular , or trait, and all the keys combining to make us who we are - then epigenetic processes determine when and how each key can be , changing the tune being played.
I
One way the study of epigenetics is our understanding of biology is by revealing a by which the environment directly impacts on genes.
Studies of animals, for example, have shown that when a rat experiences stress during pregnancy, it can cause epigenetic changes in a that lead to problems as the grows up.
Other epigenetic processes appear to occur , while others are normal, such as those that guide as they become heart, brain, or cells, for example.
J
Geneticist Danielle Reed has worked with many twins over the years and thought deeply about what twin studies have taught us.
‘It's very clear when you look at twins that much of what they share is ,' she says.
‘Many things about them are absolutely the same and .
But it's also clear, when you get to know them, that other things about them are different.
Epigenetics is the origin of a lot of those differences, in my view.'
K
Reed credits Thomas Bouchard's work for today's in twin studies.'
‘He was the ,' she says.
‘We forget that 50 years ago things like were thought to be caused entirely by lifestyle.
Schizophrenia was thought to be due to poor .
Twin studies have allowed us to be more about what people are actually born with and what's caused by experience.'
L
Having said that, Reed adds, the latest work in epigenetics promises to take our understanding even further.
‘What I like to say is that 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.'
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