h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

‘There is an association of obesity within families, and this may be due to a combination of genetics and lifestyle factors’.

Research led by geneticists from Oxford Uni (UK) and the Peninsula Medical School in Exeter (funded by the Wellcome Trust) and reported in the journal ‘Science’ (April ’07) has shown a flawed version of the FTO gene. This is the first time a specific gene has been linked to obesity.

As genes come in pairs, the people most at risk are those who carry two flawed versions of the gene. They are around half a stone heavier than average
Those who have inherited one flawed version are 30% more likely to be obese and 25% more likely to develop diabetes than those who have two normal copies
From this, the scientists say that that the genetic make-up of one in six Britons increases their risk of becoming dangerously overweight by 70%, and their chances of developing diabetes by one-half.
Prof Mark McCarthy of Oxford says that it is likely that the FTO gene is just one of several that may be linked to obesity.

http://news.bbc.co.uk/1/hi/health/6547891.stm

...and the 'eat whatever you like' gene:

Within a week of the above research being published, further research published in J.Molec. Biol. tells of a gene known as APOA5 which affects fat metabolism. It seems to enable those in possession of the gene to eat fatty foods with relative impunity as far as their weight is concerned. The research was carried out at Tufts Univ, Boston, USA and shows that those carrying the gene were much less likely to become obese than those who ate the same foods, and exercised the same amount.

... and research published in ' (June 08)Obesity' by scientists from Aberdeen Uni and the Rowett Research Inst found that volunteers with the flawed FTO gene (determined by blood testing) ate an extra 120-290 calories/day
According to spokesman Prof John Speakman, "diets are often doomed because attempts to control food intake are being driven by a deep, fundamental genetic process'.