Research into twins can help us unlock the secrets of inherited disease - and can also predict whether we will smoke, be unfaithful or even snore

"When Karina had viral meningitis I experienced similar symptoms, and I really believed I was going to die. The hotel doctor was sent for, but I think they all thought I was mad. When we got back home, we discovered that Karina had meningitis and that the worst night had been that on which I had those symptoms.''

Thousands of twins like Sonya and Karina are now part of research projects around the world that are helping to answer questions that will radically change the way almost every disease is tackled. Although it has long been accepted that disease is down to a combination of genetic predisposition and the environment, just how much of the risk of disease is inherited, and how much is down to lifestyle, has remained largely unknown.

Calculating the genetic and environmental risks has proved difficult because almost all of us have different genes and different environments. The big attraction in studying twins is that genes or environment, or both, are the same. Identical - or monozygotic - twins come from the same fertilised egg, and have identical genes, while non-identical - or dizygotic - twins have the same genetic inheritance as any other sibling. When reared together both types of twins are exposed to the same environmental influences. When reared apart, it's possible to see how the same genes behave in different environments.

"It is crucial to have the identical and the non-identical because it is the comparison of the similarity between the twin pairs that allows you to work out whether a trait is genetic or not. If both types of twins share equally similar family environments, any greater similarity between the identical twins than between the non-identical twins will be due to genetic influences," says Dr Lynn Cherkas, genetic analyst at the UK's biggest twin register at St Thomas' Hospital.

Mixing and matching the genes and lives of thousands of identical and non-identical twins means that scientists can now work out precisely how much disease risk is due to genes and how much depends on the environment.

The research also shows remarkable similarities between twins separated at birth and brought up by different familles. London-born identical twins Barbara Herbert and Daphne Goodship were separated at birth, adopted by very different families, and did not see each other again for more than 40 years. Yet when they finally did meet, not only did they look alike, they had developed the same allergies and thyroid problems, and each had miscarried in their first pregnancy. They also had the identical heart murmur, the same brain waves, and the same weak ankles. Both had also fallen down stairs at the age of 15, and had married at the same time.

Their behaviour and habits were identical, too. Both had the habit of pushing up their nose with the palm of their hand, and both had blue as their favourite colour. Both women liked black coffee, hated maths, and smoked the same cigarettes.

Major twinning centres in the UK, America, Denmark, and Australia are now producing results that will revolutionise the way diseases are treated. Not only is the research quantifying the genetic risk for myriad conditions, it is also being used to identify individual genes responsible for diseases.

By a process of elimination it can also be used to identify environmental causes: "If, for example, you take identical twins with 100 per cent the same genes and you find one had developed an illness, and the other hasn't, it must be due to different lifestyles,'' says Dr Cherkas.

So far, researchers have looked at more than 300 diseases and traits, from heart disease, cancer and arthritis to middle-aged spread, forgetfulness, infidelity and frequency of orgasm. Intriguingly, a genetic element has been found in each and every case.

But the balance between genes and environment varies hugely. Some research suggests that an identical twin whose twin has had Hodgkin's disease is 100 times more likely to develop the condition. Skin cancers, on the other hand, have a tiny genetic element and are mostly caused by lifetime sun exposure. Other diseases are more equally balanced. Some research from Denmark suggests that 50 per cent of the risk of heart disease is due to genes.

While the genetic links to disease are the most valuable, the more intriguing findings have been in the studies of traits that have linked genetic elements to the likelihood of getting married, as well as to risk of divorce, infidelity and number of sexual partners. Traits as diverse as losing keys, snoring, intelligence, exam performance and aggression also have a heritability factor.

While the findings on disease from twin research will almost certainly lead to new treatments and new ways to prevent and control disease, the new discoveries on behaviour and traits may give rise to more unease.

As Lawrence Wright, author of Twins: And What They Tell Us About Who We Are, suggests, if so much is pre-programmed, what happens during life is inevitable from the moment of conception. "We think that we are born with the potential to behave in an infinite variety of ways, and that we consciously navigate a path through life,'' he says. "But when we read about twins who have been separated at birth and reunited later only to discover that in many respects they have become the same person, it suggests that life is a charade. All we have to do is to live out the script written in our genes.''

Back at the Norfolk stately home that they and their husbands share, Karina and Sonya recount more of the similarities and events that have shaped their lives: "I was also able to predict when Sonya was going into labour with her first child. I walked around to her house, then about 10 minutes away, and when I got there she had just woken up with contractions. I knew before she did,'' she says.

The twins are part of the twin research project run by the St Thomas' Twins Register, and they believe they are two halves of a whole. "We are identical twins. We are two halves of one person, Sonya has the right side of the brain, and I the left,'' says Karina. "What I do well, Karina doesn't have any ability for, and vice versa,'' says Sonya."It is like we are one person. People like to see us together because we carry on conversations...'' "...as if it was one person talking,'' says Karina.

Need to know

HEART DISEASE

Researchers in Denmark who monitored more than 1,200 twins found that heart disease is around 50 to 55 per cent genetic.

MIDDLE-AGED SPREAD

Public health specialists at St Louis University who looked at weight changes in middle-aged male twins, claim that almost 70 per cent of the risk of middle-aged spread is genetic.

OBESITY

The heritability of obesity is substantial - up to 90 per cent - according to study into Finnish twins. Out of almost 700 twins, only 14 were found where both were not the same size. In those cases where only one twin was obese, it is thought that only one was exposed to the unknown environmental trigger that led to them becoming overweight.

LOWER BACK PAIN

Research based on the Danish Twin Registry suggests that around 40 per cent of lower-back pain can be attributed to genetics, with the remainder due to environment. A genetic predisposition accounts for 44 per cent of lower back pain in men aged 37 to 50, and 40 per cent in women aged 34 to 46.

SMOKING

Research at Virginia Commonwealth University based on 2,600 male twins shows that smoking cigarettes is largely genetic, but use of pipes and cigars is down to environmental factors.

FREQUENCY OF ORGASM IN WOMEN

"Overall, genetic influences account for approximately 31 per cent of the variance of frequency of orgasm during sexual intercourse,'' say psychiatrists at the University of Chicago who carried out a study based on around 2,000 women.

FEMALE INFIDELITY

A survey of more than 1,600 pairs of female twins shows that genetic factors have a substantial impact on how likely women are to cheat on their partner and how many sexual partners they will have.

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