Known as epigenetics, these changes are thought to occur in the womb

Distinct patterns of modifications to DNA were linked to

Scientists claim they can predict whether someone is gay or straight with up to 70 per cent accuracy by looking at their DNA.

It has long been believed that sexuality has a biological basis – with certain genes linked to being gay.

But the claim sexual behaviour can be predicted by such a high degree was described as 'bold' by British scientists.

Researchers found distinct patterns of molecules which attach to DNA to switch genes on and off were associated with homosexuality. They claim to be able to predict whether someone is gay (illustrated) or straight with 70 per cent accuracy

The findings do not show whether a test would show whether a child would grow up to be gay or not – as the research was carried out on adults.

While our genes are determined at birth – tags on the genes which affect how they work can change over time because of factors in the environment, or in the womb.

'TOMBOY GENE' LINKED TO PROMISCUITY Girls who are tomboys grow up to have more lovers whether they are straight or gay, a study has found. For it seems the genes that make women more masculine may also make them more promiscuous, researchers say. The finding helps explain why homosexuality doesn’t die out, even though lesbian couples are not able to naturally have children carrying the genes of each partner. This is because straight tomboys have more lovers than others – and so the ‘tomboy DNA’ stays in circulation, according to the study carried out by a team from King’s College London. Researchers questioned almost 500 pairs of twins about whether they were attracted to men or women, how many lovers they had had and about their levels of ‘childhood gender non-conformity’ – if they were a tomboy and whether they preferred to play with boys rather than other girls. They compared the answers from identical twins, who share all their DNA, with those of non-identical twins, who share only half their DNA. Advertisement

Dr Tuck Ngun, from the University of California at Los Angeles said: 'To our knowledge, this is the first example of a predictive model for sexual orientation based on molecular markers.'

Identical twins usually – but not always – have the same sexuality. This finding has led scientists to believe there is a genetic component to being gay.

To pinpoint the genetic areas which are linked to some people being gay, Dr Ngun and his team studied the genes of 47 pairs of adult male identical twins.

The study involved 37 pairs of twins in which one brother was homosexual and the other heterosexual, and 10 pairs in which both were homosexual.

Using a computer program called Fuzzy Forest they found that nine small regions of the genetic code played the key role on deciding whether someone is heterosexual or homosexual.

The research looked at a process called 'methylation' of the DNA – which has been compared to a switch on the DNA – making it have a stronger or weaker effect.

This process can be triggered by hormonal effects on the growing foetus in the womb.

While identical twins have exactly the same genetic sequence, environmental factors lead to differences in how their DNA is methylated.

Thus, by studying twins, the researchers could control for genetic differences and tease out the effect of methylation. This alteration of the gene is known as an 'epigenetic' effect.

The researchers, who are due to present their findings at the American Society of Human Genetics 2015 Annual Meeting in Baltimore, say they found distinct patterns of methylation on the DNA that seem to be associated with homosexuality.

The researchers examined a type of DNA modification known as methylation, where molecules attach to genes due to environmental factors while in the womb and shortly after birth

Dr Ngun said: 'Sexual attraction is such a fundamental part of life, but it's not something we know a lot about at the genetic and molecular level.

'I hope that this research helps us understand ourselves better and why we are the way we are.'

The findings, however, do not mean scientists could predict the sexuality of a child before it is born – as the tests were carried out on adults.

By pinpointing distinct genetic patterns that seem to play a role determining whether someone is gay or straight, it could conceivably be a step towards creating a test for homosexuality.

Any such test would be highly controversial – not to say unethical – as it raises the possibility unscrupulous scientists could try to decide the future sexuality of ‘designer babies’.

However, epigenetic changes tend to occur in the womb or shortly after birth due to the influence of the environment.

They can also be passed down according to the lifestyle of a person's parents and even their grandparents.

But in a further hugely controversial possibility, the findings could lead to scientists trying to alter an individual’s sexuality in adulthood by changing the methylation of the genes.

Commenting on the research, Professor Tim Spector, from King's College London, a leading expert on twin studies and genetics, said: 'It has always been a mystery why identical twins who share all their genes can vary in homosexuality.

'Epigenetic differences are one obvious reason and this study provides evidence for this. However the small study needs replicating before any talk of prediction is realistic.'

Scientists say further research will be needed to validate whether the patterns seen on the DNA of gay men (pictured) are reflected in larger populations. They say claims about predicting homosexuality are 'bold'

Darren Griffin, professor of genetics at the University of Kent, said: 'To claim a 70 per cent predictive value of something as complex as homosexuality is bold indeed. I wait with bated breath for a full peer-reviewed article.

'While there is strong evidence in general for a biological basis for homosexuality my personal impression has always been one of a multiple contributory factors, including life experiences.'

Gil McVean, professor of statistical genetics at Oxford University, said: 'Without validation of the result in an independent data set it is not really possible to know whether there is any substance in this claim.'

Dr Christopher Gregg, Assistant Professor of Neurobiology & Anatomy and Human Genetics, University of Utah said:

'Overall, the importance of these findings will hinge on how reproducible they are in future studies that include larger groups of heterosexual and homosexual individuals.'

Gay and human rights campaigner Peter Tatchell said: ‘This research claims to be able to predict sexual orientation with 'up to' 70 per cent accuracy. This doesn't sound entirely convincing or reliable.

'Some people fear the research might be abused by homophobic parents or regimes to test and abort foetuses that have genetic markers for homosexuality.

‘Even if the tests achieved the maximum 70 per cent detection rate, it would still leave 30 per cent undetected.