It's official: size is all down to your genes

Scientists have uncovered the genes which determine the size of your extremities - from your fingers to your genitalia - and others which could decide how quickly you appear to age. Charles Arthur, Science Editor, explains.

Jokes about big-footed men casting a longer shadow in bed might actually have some basis in fact. An international team at the University of Geneva in Switzerland, and the University of Michigan's Medical School has pinpointed the family of genes which determine how big your hands, feet, and even genitals will be.

Known as the Hox genes, they seem to be the dividing line between us and our most distant aquatic ancestors, because it is legs and arms which principally distinguish us from fish.

The team found that the embryos of genetically-engineered mice which lacked particular versions of the dozen or so Hox genes developed without digits or penises. (The mice died before birth.) That suggests that the Hox genes are closely concerned with the development of our extremities. If that is the case, then when one extremity, modulated by the Hox genes, is large, then the others should be too because their development is driven by the same gene.

But that factor also affects genitalia, for the possession of digits and external genitalia are the factors particularly distinguishing land- based animals from aquatic ones - suggesting that the Hox genes evolved in response to the need to move around on dry land. Writing in the science journal Nature, the researchers at the National Institute of Neuroscience suggest that internal fertilisation may have developed at the same time, since the terrestrial environment makes external fertilisation almost impossible compared to an ocean-based one. That could tie the Hox genes for all those extremities together.

But while the development of those genes lies tens of millions of years in our past, many people are now looking to science to offer hope for the future: specifically, to make them live longer, or at least help them appear to. Again, mice have had to stand in for humans. A Japanese team, also reporting in Nature, now reckons that a newly-discovered gene could play a part in how quickly one appears to age.

With cosmetic companies frequently promising to "slow down the appearance of ageing" with expensive oils, the concept that such a task could be tackled by genes instead may seem surprising. But the new gene, dubbed klotho (after one of the Fates, a Greek goddess who spins the thread of life), seems to be involved in a complex chemical and genetic pathway, which both regulates ageing and may influence your susceptibility to age- related diseases.

Mice with two faulty copies of the klotho gene grew and aged normally at first, but then aged rapidly and died of various illnesses including arteriosclerosis, osteoporosis, and emphysema, after about 60 days - one-tenth the usual lifespan of a laboratory mouse. From the experiments, it seems that the klotho gene is most effective as a regulator of the normal ageing process, ensuring that it does not run out of control.