Steve Connor: How Trinny and Susannah defy weight expectations

The fact that body weight has a strong inherited component is obvious to anyone, but exactly how genes and environmental upbringing interact with one another has remained something of a mystery.

Fat parents are more likely to rear overweight children, but this could be because of the type or amount of food that is eaten in the family rather than the genes that are inherited. Nevertheless, body weight has strong "heritability", meaning that some people are just born with a set of genes that predispose them to being overweight, especially when they are raised in a high-calorie environment.

But equally, the many genes that influence body weight can interact in a complex and unpredictable way that sometimes defies scientific explanation. The television presenters Trinny Woodall (the slim one) and Susannah Constantine (the buxom one) illustrate this problem perfectly.

When the two presenters took part in a TV programme on the Great British Body, both women undertook a DNA test looking for variants of the FTO "fat" gene. To the surprise of the two presenters, it was Trinny who was carrying the "fat" variant, while Susannah had inherited the "slim" FTO variant.

This shows that even though population-wide studies on hundreds of people have shown beyond doubt that the "fat" variant of FTO gene is associated with a significantly increased risk of being overweight, the statistical association does not automatically mean that everyone with that "fat gene" is going to be overweight.

Evidently, there must be other factors in play that keep Trinny and Susannah in the shape they come in. Trinny may have inherited other genes that keep her "fat" FTO gene in check, or she may have been raised in an environment that had the same effect – or she could have incredible will power to defy her appetite-enhancing gene.

For Susannah, some of her other genes are probably working against the benefits of the slimline variant of FTO. But it could be that without this particular gene she might have become even more buxom.