People have asked me if their body fatness is genetic, to which I reply unhelpfully, yes and no. There is a very tiny proportion of any population that very easily puts on weight, where a small number of genes relate to, or are implicated in, excessive weight gain. There is a larger group of people with several genes that are triggered by one or several of a range of environmental factors to gain body fatness. And then there is almost everyone else, each person with thousands of genetic variants that have seemingly small individual effects in relation to obesity under circumstances that favour it. There are also those annoying people who can eat as much as they want, of anything they want, and not put on weight, seemingly with genes for thinness. The study of obesity genetics is steadily letting the genie out of the bottle – once you know something, you can’t unknow it, and we know enough about this area now to be able to reply with ‘It depends’, when someone says, ‘My body fatness is down to my genes.’