On the day we each spit into separate test tubes, I don’t yet understand how a DNA test can offer evidence of compatibility, because I am only on page eight of Daniel M Davis’s book The Compatibility Gene. But here’s the gist of the idea: there are a small number of human genes � a tiny section of the short arm of chromosome six � that may play a role in determining how attractive you are to a potential mate.
And while the mechanism behind this phenomenon is poorly understood, that hasn’t stopped dating agencies from employing MHC typing as a matchmaking tool
The basis for this notion is the so-called smelly T-shirt experiment, first performed by a Swiss zoologist called Claus Wedekind in 1994. He analysed a particular bit of the DNA of a group of students, looking specifically at the major histocompatibility genes (MHC). The students were then split into 49 females and 44 males. The men were asked to wear plain cotton T-shirts for two nights while avoiding anything � alcohol, cologne etc � that might alter their natural odour.
Wedekind’s results appeared to show that the women preferred the T-shirts worn by men with different compatibility genes from themselves, raising the possibility that we unconsciously select mates who would put our offspring at some genetic advantage. The experiment was controversial, but it did alter scientific thinking about compatibility genes. One lab offering such testing to online agencies (you can’t smell potential partners over the internet; not yet), a Swiss company called GenePartner, claims: “With genetically compatible people we feel that rare sensation of perfect chemistry.”
After two days 
the shirts were placed in cardboard boxes with holes in them, and the women were asked to rank the boxes by smell using three criteria: intensity, pleasantness and sexiness
As I walk to the postbox with my two test tubes of spit in an envelope, the idea of testing my genetic affinity with my wife suddenly strikes me as foolhardy. Twenty years of marriage should be the very definition of compatibility, but what if the results tell a different story? I don’t want to discover that on a cold winter’s night two decades ago, my wife took one sniff of me and fell in love with my deodorant. I don’t think they even make that kind any more.
Davis also tested his marital compatibility for the book and, while he may be a director of the University of Manchester’s Collaborative Centre of Inflammation Research, he admits to similar, not wholly rational, misgivings.
“It was definitely more weird than I thought,” he told me, adding that his wife was “unexpectedly nervous about what they might find.” He needn’t have worried � they were pronounced perfectly compatible.
They aren’t called your compatibility genes because they help you find a compatible partner; they’re called that because they govern the acceptance and rejection of transplanted organs. And that’s not their intended role, either. As Professor Steven Marsh � deputy director of research at the Anthony Nolan Histocompatibility Laboratories, where I sent my spit � puts it: “The molecules that give you your tissue type, they’re not there just to make transplantation difficult. Their job is to fight infection.” They are, in short, your immune system.
Davis’s book tells the story of the search for these compatibility genes, from the early days of blood transfusion to the cutting-edge science that has yet to appear in the textbooks. “I kind of wanted to step back and take in the big picture,” he says. “You can quite easily have a successful career in science without knowing how you got where you are.” As a journalist and a layman I am normally happy to summarise decades of tireless research with the words, “It’s complicated”, but some further explanation is warranted.