An estimated 700,000 people in the UK are morbidly obese
A small number of extremely overweight people may be missing the same chunk of genetic material, claim UK researchers.
The findings, published in the journal Nature, could offer clues to whether obesity can be "inherited" in some cases.
Imperial College London scientists found dozens of people - all severely obese - who lacked approximately the same 30 genes.
The gene "deletion" could not be found in people of normal weight.
While much of the "obesity epidemic" currently affecting most Western countries has been attributed to a move towards high-calorie foods and more sedentary lifestyles, scientists have found evidence that genes may play a significant role in influencing weight gain in some people.
The latest study focused on the "morbidly obese", who have a Body Mass Index (BMI) of more than 40, and who are at the highest risk of health problems.
There are an estimated 700,000 of these people in the UK.
The first clue came by looking at a group of teenagers and adults with learning difficulties, who are known to be at higher risk of obesity, although the reasons for this are not entirely clear.
They researchers found 31 people who had nearly identical "deletions" in their genetic code, all of whom had a BMI of over 30, meaning they were obese.
Then a wider scan of the genetic makeup of a mixture of more than 16,000 obese and normal weight people revealed 19 more examples of the missing genes.
All of the people involved were classed as "morbidly obese", with a BMI of over 40, and at the highest risk of health problems related to their weight.
Most of them had been normal weight as toddlers, but then became overweight during later childhood.
None of the people studied with normal weight had the missing code.
The precise function of the missing genes is unclear, as is the precise nature of the relationship between learning difficulties and obesity - none of the people with the deletions in the wider study had learning problems.
Professor Philippe Froguel, from Imperial College, said: "It is becoming increasingly clear that for some morbidly obese people, their weight gain has an underlying genetic cause.
"If we can identify these individuals through genetic testing, we can then offer them appropriate support and medical interventions, such as the option of weight loss surgery, to improve their long-term health."
Dr Robin Walters, also from Imperial, said that while this particular set of deletions was rare - affecting some seven in 1,000 morbidly obese people - there were likely to be other variations yet to be found.
"The combined effect of several variations of this type could explain much of the genetic risk for severe obesity, which is known to run in families."
Dr Sadaf Farooqi, from Cambridge University, who collaborated with this research, and was involved in similar research published in December which pointed to another gene flaw which could be linked to obesity.
She said it was likely that a "patchwork" of different genetic variations would eventually emerge to explain more cases of obesity - perhaps by affecting appetite, or the rate at which the body burns fat.
She said: "There is still an important public health message about diet and exercise, but simply blaming people for their obesity is no longer appropriate."