Ooh Sexy Genes! Our Immune System Genes, that is.

Stop looking at everyone's breasts, pecks or derriere. For goodness sake, this is the 21st Century! Surely you know that these are highly unreliable cues as to who might be The One. Maybe it's time to get to the heart of the matter - and think about genes. Oh, you were looking for that old chestnut - a Good Sense Of Humor? Well sure, laughter helps, but after a few years you'll have heard all their jokes (more than once, believe me) - and then what? Surely a bunch of good genes is what you're really after when you're looking for someone to live with, a soul-mate, someone to have kids with?

So the question is: are genes important in relationships and if so, which genes; surely not those that control our hair color, eye color - or the shape of your backside?

It turns out that actually, the biggest genetic difference from one person to the next has nothing to do with looks. Out of the 25,000 genes that make up a human, there's one small cluster of genes which are exceptionally variable between each of us - called our Compatibility Genes. These genes vary so much that they are in effect a mark of our individuality - our uniqueness, on a molecular scale. In one database of 18 million people, for example, just four had versions of these genes similar to me.

These are the genes that doctors try to match in many types of medical transplantation - bone marrow transplants for example - to give the best chance of the operation being a success. But of course, the real job for these genes can't be to cause difficulties for transplant surgeons. So what do these genes really do in the body? The answer has been uncovered through over sixty years of scientific research - a global adventure with a cast of thousands - that amounts to a scientific revolution in understanding how the human body works.

This vast human endeavor has revealed that, first and foremost, these genes are critical to how our bodies fight disease. We know how this works in great detail. And we know now that our differences in these genes are one reason why you may recover from a viral infection quicker than someone else who's been infected with the exact same virus.

Our battle with infections is important, but the story of these genes is bigger still - and is controversial. While mainstream science focused on how these genes work in the immune system, others - by and large, working away from the limelight - have found evidence that these same genes can also influence sexual attraction.

There's good evidence for this in some animals. To test this, an animal, say a mouse, is placed at the bottom of a Y-shaped tunnel so that it can then choose to run down either prong of Y to choose a mate. These kinds of experiments showed that mice prefer to mate with other mice that have different compatibility genes to themselves. Something of this may be true in humans. There's some evidence that women prefer the smell of T-shirts worn by men if the men have different versions of compatibility genes to themselves. Very recently, these genes have also been shown to correlate with the likelihood of couples having particular problems in pregnancy.

There are two ways of looking at this. One: the body just so happens to use the same genes for different things. Or two: these different aspects of human biology - reproduction and survival from disease - are fundamentally linked. We don't yet know the answer for certain, but I think a fundamental connection is more likely. It would be no good if one sweeping infectious disease that killed people with certain versions of these genes would simply narrow the variation in these genes that got passed on to the next generation and lower our chances against other diseases in the future. A role for these genes in reproduction would provide a second process to influence which versions of genes get passed on - help to keep these genes diverse. And for the way our body fights disease, it is beneficial to keep these genes exceptionally diverse.

This science has a powerful message for society (listen up, racists); there is no hierarchy - nobody has a perfect set of compatibility genes - our diversity is what's important. Each of us will be better or worse at fighting off particular germs - and the way our species has evolved to survive disease requires us to have this variation.

So these are the genes important to our relationships? Scientists argue over the impact of these genes outside the immune system; they may just play a very small role in our relationships, because - while mice sniff each other's urine to choose a mate - human interactions are blatantly more complicated. But either way, compatibility genes are part of your genetic identity - and our understanding of what they do provides a new perspective to the wonder of who you are; a unique component in a vast genetic system built to fight disease.