What makes a snail veer to the left? Photo: GETTY

I once tried to make a pair of earrings from two snail shells, each a mirror-image of the other. The mollusc I have spent (or wasted) my career on comes, almost always, with the opening on the right hand side of the shell. Very occasionally – perhaps once in 10,000 times – a reversed, or “sinistral”, individual turns up, giving the opportunity for earrings. I have collected a dozen or so over the years, although I have no doubt failed to notice many more. Rare as the lefties are, in some tropical species both sorts live together as a matter of course, which leads to awkward fumbling when sex rears its head.

Why should a mollusc – or anything else – veer to the left (or for that matter to the right)? Now the culprit has been identified. It is a gene called nodal, which is active on the right side of the embryo of right-coiling snails, and the other side in the leftists. The mutation does nothing to the animal’s own shell, but affects her eggs instead and alters the direction of twist of her offspring. The pattern of inheritance is hence delayed by one generation.

Plants, too, can spiral. Comic singers Flanders and Swann warned the (clockwise) honeysuckle of the danger of sex with the (anticlockwise) bindweed: what would their infant do? “Poor little sucker, how will it learn/When it is climbing, which way to turn? Right – left – what a disgrace! Or it may go straight up and fall flat on its face!”

In plants, the framework of every cell is built of a certain protein whose units slot together into long hollow cylinders. In a mustard much used in genetics, a mutation called “lefty” enlarges one end of each building block and causes the normally upright stem to spiral to the left, while another version does the opposite. Plants with a single copy of each do indeed grow upright. Much the same happens in snails, for drugs that inhibit their twist genes generate animals that lack any coil and grow straight upwards.

And what about us? We have plenty of asymmetries of our own: in almost everyone, the heart is on the left side and the liver on the right. And, as at least half of the population knows, there is also an interesting lopsidedness in the male genitalia. Sometimes, though, those patterns disappear. In around one person in 10,000, the heart is on the right; and in even fewer the whole contents of the abdomen are a mirror image of normal. Once again, the nodal gene is to blame, for we share it with our molluscan friends, and a damaged version of that piece of DNA is responsible for some of the rare and spectacular shifts in heart or gut position.

Just as in the mutation found in plants, the altered gene interferes with the directional twist of the hollow hairs that cover the cells of the very early embryo. Their normal movements help shape the growing foetus, but when they lash in the wrong direction they somehow remove its normal symmetry. As a result, the hearts of half of those inheriting the inborn error swing – as normal – to the left as they develop, but those of the other half bear away to the right and invert their insides.

Although one gene is responsible for some of the twists of snails and humans, there is still plenty to find out. About one person in 10 is left-handed, but how that pattern is inherited is far from clear (although Oxford scientists have found a DNA variant that predisposes to it). There is also the odd observation that certain chemicals damage a growing embryo on the left side, and others on the right. Another unexpected discovery is that identical twins are often mirror images of each other, at least above the neck. The hair whorl on the back of the head often twists one way in one member of the pair, and another in the twin. In the same way, if one has a single wisdom tooth on the left, the other tends to have theirs on the right.

My mother was an identical twin and showed that mirror pattern when compared with her sister. I sometimes claim to students that this is why I became a geneticist. That is not true, but mothers do affect the spin of their offspring in several ways.

Some American scientists claim that even the party one votes for, on the Left or the Right, is influenced by inherited variation in the DNA that codes for a certain brain receptor. My mother, though, was a lifelong Conservative, while I tend to spiral in the opposite direction.

• Steve Jones is professor of genetics at University College London. His latest book ‘Darwin’s Island’ (Little, Brown) is available from Telegraph Books for £18 + £1.25 p&p. To order, call 0844 871 1515 or go to books.telegraph.co.uk