Women who inherit a gene that ups the risk of obesity also run a higher risk of fertility problems due to polycystic ovaries, experts have discovered.
Half of white Europeans carry a copy of the FTO variant - which is the clearest genetic clue we have to explain why some people are prone to weight gain.
Now experts from Oxford and Imperial College London believe the same gene may cause polycystic ovary syndrome.
The results were presented at an annual endocrinology meeting in Harrogate.
Dr Tom Barber and his team analysed the type of FTO gene carried by 463 patients with polycystic ovary syndrome (PCOS) and 1,336 other women.
They found that the FTO version already dubbed the "fat gene" was far more common in the women with PCOS, and particularly among those who were also overweight.
Experts already know that PCOS is linked with obesity and this new discovery provides genetic evidence for the link.
Dr Barber said: "PCOS is an incredibly common condition affecting one in 10 women of reproductive age and is a leading cause of infertility.
"It is a genetic condition and one that is strongly associated with obesity; it is therefore of huge relevance for women given today's obesity epidemic.
"Our research shows that a variant of the FTO gene that has previously been shown to be associated with obesity also influences susceptibility to polycystic ovary syndrome.
"These data provide the first genetic evidence to corroborate the well documented association between these two conditions."
Although not all women with PCOS are overweight, Dr Barber believes obesity is the underlying problem and may trigger disturbances in hormones including testosterone and insulin.
"There is no doubt that women who gain too much weight are more likely to develop features of PCOS and even small amounts of weight loss can make a real difference to the condition.
"The FTO gene does seem to be important, but it is likely to be one of a number of genes implicated in PCOS and obesity. We plan to do a genome-wide study to find the complete set."
The ultimate aim is to assess an individual's risk and provide targeted therapy, whether that is correcting the faulty genes or ensuring women with a risky genetic make-up pay particular attention to their lifestyle, said Dr Barber.
Mr Laurence Shaw of the Bridge Fertility Centre in London said it was possible that the gene was an evolutionary throw back from times of famine.
"Women with PCOS have a more efficient basic metabolism. They do well on fewer calories but are at a disadvantage in these well-fed times that we are in now.
"Discovering genes like this will help us better diagnose the condition and predict what interventions are needed."