The old adage that being overweight "is in the genes" may be true, at least partially, for some, say researchers.
Some obese people may be carrying the faulty gene
A team from the University of Marburg in Germany found mutations in the MC4R gene had a "major" impact on people's body mass index.
BMI is weight in kilograms divided by the square of your height in metres.
In the Journal of Medical Genetics, the researchers say people who have the mutations are significantly heavier than those who do not.
Rates of obesity amongst UK adults have almost quadrupled in the last 25 years. About 22% of Britons are now obese.
Around 10% of six-year-olds are believed to be obese, rising to 17% when 15-year-olds are considered.
Obesity is likely to be one of the issues covered in the forthcoming Public Health White Paper, due to be published by the government this autumn.
The German researchers examined the body weights and DNA of 181 relatives of 25 extremely obese patients who carried the gene mutation.
There are up to 34 MC4R mutations, although only 2% to 3% of very obese people carry them.
It was found that carriers of the mutations had a significantly higher BMI than those who did not.
The effect was about twice as strong in women as in men.
In women the mutations accounted for 9.5 kilogram per metre² additional weight in middle aged women and for 4 kg in middle aged men.
The team said that the mutations were linked to a "strong predisposition" to obesity.
And, while rates of obesity dropped substantially between first and second degree relatives of obese patients without the gene mutations, this was far less obvious among relatives of those who were carriers.
The researchers said this provided more evidence for the impact of the mutations.
Writing in the journal, they said: "It is well known that body weight is influenced by many genetic and environmental factors.
"MC4R mutations entail a strong predisposition to obesity," conclude the authors.
But they add that the high rate of body fat among the relatives of the carriers suggests that the mutations do not, by themselves, account for obesity.
They said that the high rate of body fat among the relatives of carriers
of the mutated gene suggested that mutations by themselves did not account for
In addition, they said other genetic and environmental factors play their part, shown by the fact that thin people can also carry the genetic mutations.