If this wonder protein were present in humans, mythical Greek king Oedipus could have avoided an embarrassing situation. It would have helped the legendary king of Thebes know that Jocasta — whom he married and had children by — was actually his mother, and thus perhaps averted a travesty of fate.

The common house mouse, however, would never find itself in such an awkward position. For the rodents are endowed with a set of proteins that help them identify their close kin and thus avoid committing incest.

These proteins are present in their urine and are called major urinary proteins (MUP). The proteins actually serve as a “chemical bar code of individual identity” with each mouse having a slightly different set of proteins than others, says Jane Hurst, professor of mammalian behaviour and evolution at the University of Liverpool, US, who led the team which figured out the functions of these specially evolved substances.

Wild mice with the same sets of proteins, or those with very little variation, can also recognise relatives and thus avoid mating with each other, Hurst says. By simply checking the match between their own urine proteins and those of the animal they meet, they are able to identify many of their closest relatives, even if they have never met before. But “it is not a perfect system,” says Hurst.

Mating between close relatives is avoided in many animals because it can result in shared faulty, otherwise hidden (or recessive) traits surfacing in the offspring, a phenomenon known as inbreeding depression.

The finding, reported last week by Hurst and her colleagues in the online version of the journal Current Biology, is significant for another reason: it challenges a widely-held assumption that another set of genes — called major histocompatibility complex (MHC) — plays a key role in helping animals to avoid inbreeding. The scientists found that this is not true, at least in the case of wild mice.

MHC, a set of genes crucial to the immune system, was so far thought to play an important role in scent recognition in vertebrates. It is an extremely variable group of genes that allows the immune system to identify and defend the body against foreign invaders. Each individual has a different MHC code, just like every human has a unique fingerprint. It also influences body odour and it was assumed that animals that are sensitive to scent would use these different odours to recognise each other.

But now, scientists have discovered just the opposite of what had been found in earlier studies.

The use of MUP alone is sufficient to explain inbreeding avoidance, the researchers write in their paper.

Studies in the past in humans and laboratory mice have shown that members of the opposite sex with a different MHC code were “more preferable”. A classic example of this was the T-shirt sniffing study conducted by Swiss behavioural ecologist Claus Wedekind in the 1990s. When Wedekind’s team asked women — who were in the potentially fertile phase of their ovarian cycle — to assess the body odours of men by sniffing their smelly T-shirts, it was found that the odours of men with a dissimilar MHC were more pleasant (or at least less unpleasant) to them than those of men with MHC genes similar to theirs.

Hurst’s is an interesting piece of work as “it overturns long-accepted ideas about the role of the MHC in avoiding inbreeding,” Upinder Bhalla, a scientist at the Bangalore-based National Centre for Biological Sciences, told KnowHow. Bhalla, who thinks similar proteins are likely to be found in other animals too, is studying smell-related brain functions in mice.

“To my knowledge, it is for the first time that other proteins (apart from MHC) have been identified to have a role in helping animals to avoid inbreeding,” remarks Bhalla.

Interestingly, mice were subjected to similar smell studies as those conducted by Wedekind. And these animals were also found to prefer mates with MHC genes different from their own. But most of these studies were conducted in artificially inbred, laboratory mice that were genetically identical except for their MHC genes.

But what did help Hurst and her team was the fact that they chose wild mice instead of laboratory-bred animals. They also used parentage testing on the offspring to determine which animals had mated with each other.

Unlike wild mice, those bred in the laboratory do not have individually variable MUP patterns in their urine. This is perhaps the reason laboratory-bred mice looked for a different MHC in their mates. In a natural environment, the situation is completely different, says Hurst. In wild animals, the MHC make-up is not the only thing that is found to differ from animal to animal. “In natural populations, animals have to be able to recognise MHC scents against the genetic variation of normal animals — not when this is the only difference between them,” Hurst told KnowHow.

Significantly, the UK scientists also found that the wild mice they used did not totally avoid mates that had the same MHC type as themselves, indicating a limited role played by the MHC in a natural environment.

But negating the role played by the MHC in avoiding inbreeding would be too broad a conclusion to draw from a single experiment, says Hurst, particularly when there is some evidence to show that female stickleback fish prefer mates with different MHC genes, because this will increase the diversity of MHC genes in her offspring. And MHC diversity is important for marshalling defences against parasites.

After all, it is a natural aspiration of all living creatures to bring forth healthy offspring.