Around one in 500 men could be carrying an extra X or Y chromosome - most of them unaware - putting them at increased risk of diseases such as Type 2 diabetes, atherosclerosis, and thrombosis, according to a study.

Sex chromosomes determine our biological sex. Men typically have one X and one Y chromosome, while women have two Xs. However, some men also have an extra X or Y chromosome - XXY or XYY.

Without a genetic test, it may not be immediately obvious. Men with extra X chromosomes are sometimes identified during investigations of delayed puberty and infertility; however, most are unaware that they have this condition.

Men with an extra Y chromosome tend to be taller than boys and adults, but otherwise, they have no distinctive physical features. The study published in the journal Genetics in Medicine showed that men with XXY have much higher chances of reproductive problems, including a three-fold higher risk of delayed puberty and a four-fold higher risk of being childless.

These men also had significantly lower blood concentrations of testosterone, the natural male hormone. Men with XYY appeared to have a normal reproductive function.

Men with either XXY or XYY were also three times more likely to have Type 2 diabetes, six times more likely to develop venous thrombosis, three times as likely to experience pulmonary embolism, and four times more likely to suffer from the chronic obstructive pulmonary disease (COPD).

Researchers at the Universities of Cambridge and Exeter said that it isn't clear why an extra chromosome should increase the risk or why the risks were so similar irrespective of which sex chromosome was duplicated.

"Even though a significant number of men carry an extra sex chromosome, very few of them are likely to be aware of this. This extra chromosome means that they have substantially higher risks of a number of common metabolic, vascular, and respiratory diseases - diseases that may be preventable," Yajie Zhao, a doctoral student at the Medical Research Council (MRC) Epidemiology Unit at Cambridge.

"We'd need more research to assess whether there is additional value in wider screening for unusual chromosomes in the general population, and this could potentially lead to early interventions to help them avoid the related diseases," Professor Ken Ong, also from the MRC Epidemiology Unit at Cambridge and joint senior author, added.

The team analysed genetic data collected on over 200,000 UK men aged 40-70. They found 356 men who carried either an extra X chromosome (213 men) or an extra Y chromosome (143 men). As the participants tend to be 'healthier' than the general population, this suggests that around one in 500 men may carry an extra X or Y chromosome, they said.

Previous studies have found that around one in 1,000 females has an additional X chromosome, which can result in delayed language development and accelerated growth until puberty, as well as lower IQ levels compared to their peers.