By Dr Alexander van Tulleken
Presenter, The Secret Life of Twins
Twin brothers, but not quite identical
My identical twin brother Chris is 2cm taller than me.
Barely noticeable you would think. I can see what it is like to be him by standing on tip-toes just a little and frankly the world does not look much different from up there.
Nonetheless it bothers me: I could have been that tall. I have exactly the same genes as my brother.
Genes that, in the right environment, could have made me a full 185cm tall as opposed to my current 183cm.
Something, somewhere went wrong and I got stunted. Not by much but it is a reminder of what could have been.
And that is the real problem.
It is not the height, not that trousers fit him a little better or that he does not need to reach as far to change a light bulb.
It is that I wonder how many other things I am missing out on.
Ten points of IQ here, some grey hair there. Perhaps I will need glasses at age 35 instead of 40.
Identical twins Chris and Alexander took part in a pain tolerance experiment
Most people do not have any way of knowing whether or not they are getting the most out of their genes but if you have a twin sibling then you have an exact comparison.
So how did these differences arise?
The differences between identical twins - people who should be exactly the same - are becoming increasingly useful to medical researchers interested in everything from mental illness to our ability to do maths.
Long term changes
Most people are used to thinking of our DNA as a fairly fixed code, a bit like a blueprint for a building.
We know that some things can change the code itself - exposure to radiation can cause mutations that lead to an increased risk of cancer for example - but usually the code remains the same.
Finnish twins Miia (left) and Noora (right) are 24-years-old
Noora weighs 17kg more than Miia due to their different lifestyles
But Miia is also likely to put on weight if she becomes less physically active
However, production of the molecules for which our genes are responsible - things like digestive enzymes and muscle protein - is constantly getting switched on and off.
And it seems that some environmental influences can have much more lasting effects - permanently activating or inactivating certain genes.
It is becoming apparent that this aspect of genetic control - a process dubbed epigenetics - is very important in human health.
In 1944 there was a severe famine in Holland. The children born during this period are more susceptible to diabetes, obesity and heart disease compared to siblings who were older during the famine.
It seems that the period of starvation prompted their bodies to switch certain genes on or off to cope with the lack of food.
These changes may, in subsequent times of plenty, have made them gain weight and get diabetes.
This change in the particular genes we express is controlled chiefly through a process called "methylation" in which chemical groups are attached to the DNA molecule to tell the body whether or not to use the "methylated" genes.
It is one of the ways in which our bodies' attempt to adapt to changing environments.
Twins play a key role in discovering more.
In the summer of 2009 Chris and I joined the twin research study at St Thomas's Hospital where we were investigated in great detail.
They measured our height, weight, bone density, grip strength even ability to hear if a nursery rhyme is in tune.
All these are traits that are to some extent genetic.
It turned out that we are different in more ways than height.
For a start I was 15kg heavier than my brother.
What about my missing 20mm of height? It is probably an epigenetic effect caused by some long forgotten environmental difference when we were little
Of the thousands of twins enrolled in the research program only 10 were more different in weight than us.
Professor Tim Spector who leads the research unit was frank: "You're (he meant me) a disgrace."
My excess weight, unlike my brother's excess height, is easily explained.
I have been living in America and eating too much. So far no surprises.
But environmental exposure to a lot of cheap, high-fat food in America may have caused more than temporary weight gain.
It may have permanently altered the way my genes are expressed.
Scientists are only just beginning to understand these processes but it is possible that I may have permanently altered my metabolism to accommodate those extra pounds: the health effects could last a long time.
Key role in disease
Epigenetics seems to play a role in a large number of diseases.
Twins studies show us that some cancers are not caused by DNA mutations but by epigenetic changes, which potentially offers new targets for drugs.
Similarly with heart disease or diabetes we might one day be considering therapies that change our epigenetics.
This is a long way off but twin studies are beginning to shed new light on how our bodies react to the environment.
What about my missing 20mm of height? It is probably an epigenetic effect caused by some long forgotten environmental difference when we were little.
I will never get it back but if it is of use to medical science I can live without it.
The Secret Life of Twins is on BBC One on Wednesday 30 September at 2100 BST.
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