One in 500 men may carry an extra sex chromosome (most without knowing it)

As many as one in 500 men may carry an extra sex chromosome — either an X or a Y — but very few of them likely know about it, a new study suggests.

The research, published June 9 in the journal Genetics in Medicine, included data from more than 207,000 men who provided information to the U.K. Biobank, a repository of genetic and health data from half a million U.K.-based participants. Typically, males carry one X- and one Y-shaped sex chromosome in each of their cells, but among the study participants, there were 213 men who carried an extra X chromosome and 143 that had an extra Y. 

Very few of these men either reported being diagnosed with a chromosomal abnormality or had such an abnormality noted in their medical records: Of the XXY men, only 23% had a known diagnosis, and just 0.7% of the XYY men had a diagnosis. (The potential symptoms of having an extra Y chromosome can be very subtle, which may somewhat explain the difference in diagnosis rates, according to the Genetic and Rare Diseases Information Center.)

"We were surprised at how common this is," Dr. Ken Ong, a pediatric endocrinologist in the Medical Research Council (MRC) Epidemiology Unit at the University of Cambridge and a co-senior author on the study, told The Guardian. "It had been thought to be pretty rare." 

Previous estimates suggested that roughly 100 to 200 men out of every 100,000 are XXY, according to the National Human Genome Research Institute, and an estimated 18 to 100 out of every 100,000 were thought to be XYY, the authors noted in their report. 

Related: Is the Y chromosome dying out? 

In all, about 0.17% of the study participants had an extra sex chromosome, or about one in 580. However, the rate observed in the study might be slightly lower than that among the general population, the study authors noted in their report. That's because U.K. Biobank volunteers tend to be healthier than the general population and have a lower-than-average incidence of genetic conditions. Based on this, the authors estimate that about one in 500 men, or 0.2%, in the general population carry an extra sex chromosome. 

Having extra sex chromosomes can raise the risk of certain health conditions, and this increased risk seemed to be reflected in the Biobank volunteers' health data, the researchers reported.

For instance, Klinefelter syndrome (KS) — or having an extra X chromosome as a male — has been linked to reproductive problems, including infertility and delayed puberty, according to the National Human Genome Research Institute. In the study, XXY men's rate of childlessness was four times higher than that of XY men, and they were three times more likely to have started puberty late, according to a statement. 

A condition called 47,XYY syndrome — or having an extra Y chromosome as a male — was not linked to an increased rate of reproductive problems in the affected study participants, the authors reported. That said, in the past, the syndrome has been linked to other symptoms, including learning disabilities, delays in acquiring speech and motor skills, and unusually low muscle tone, according to the Genetic and Rare Diseases Information Center. These symptoms were not specifically assessed in the Biobank study.

Related: Are you genetically more similar to your mom or your dad?

However, the research did reveal a possible link between extra sex chromosomes and other conditions. Compared with XY men, both the XXY and XYY men showed higher rates of type 2 diabetes; plaque build-up in the walls of the arteries (atherosclerosis); blood clots in the veins (venous thrombosis) and lung arteries (pulmonary embolism); and chronic obstructive pulmonary disease, which obstructs airflow to the lungs. 

"It is unclear why both KS and 47,XYY should show striking similarities in conferring substantially higher risks for many diseases in common," the authors wrote in their report. The mechanisms driving this increased risk will have to be explored in future studies, they said.

The study is limited in that it only included men of European ancestry who were between the ages of 40 and 70. However, "our study is important because it starts from the genetics and tells us about the potential health impacts of having an extra sex chromosome in an older population, without being biased by only testing men with certain features as has often been done in the past," Anna Murray, an associate professor of human genetics at the University of Exeter Medical School and co-senior author of the study, said in the statement.

Originally published on Live Science.