Cannibalism and Kuru Disease : Prion Disease

Do pigs share 98 per cent of human genes?
Some pigs and humans are more alike than you realize.

I heard that pigs share 98 per cent of human genes. Is this correct?
—Wendy

Like it or not, we've all got a lot in common with pigs. We're omnivorous mammals that gain weight easily and are susceptible to the flu for starters.

The sheer fact that pigs and humans are mammals means that we share some genes. But it is simplistic to put an actual figure on the amount of genetic material we have in common, says animal geneticist Professor Chris Moran from the University of Sydney's Faculty of Veterinary Science.

"Making broad comparisons by saying … 98 per cent of [human] genes are similar to a chimpanzee or whatever else … tend to be a little bit misleading," says Moran.

The amount of genetic material we share with other species depends upon what you compare.

All living organisms have genetic information encoded in deoxyribonucleic acid (DNA), divided into units called genes. Information is transferred from the genes via a chemical called ribonucleic acid (RNA). Some RNA is translated into chains of amino-acid that make up proteins, the building blocks of every living cell.

Scientists have discovered about 20,000 mammalian genes that encode proteins with similar basic functions. So if you compare the protein-encoding portion of our DNA we have a lot in common with a lot of mammals.

"Mammals have most of the same genes for similar biochemical and physiological functions. If you look at the details of the genes … there'll be differences between them, but they'll still be doing the same kind of function," says Moran.

"It's a little bit like having a Ford or a Holden — it's still obviously a car but a slightly different version."

But while 20,000 similar genes sounds like a lot, only one to two per cent of our DNA actually encodes proteins. Most of the rest is transcribed into RNA.

Some RNAs that don't carry the plans for proteins have important structural or functional roles in their own right. Transfer RNAs, for example, ferry specific amino acids into a growing protein, while ribosomal RNA constitutes part of the factories in cells that manufacture proteins.

But we are only just beginning to understand what many other non-coding RNA molecules do. Some control higher level functions such as the expression of protein-encoding genes, and some have even been implicated in memory.

Evolutionary differences

Parts of the genome that don't encode proteins tend to evolve rapidly, so you can have significant regions of the genome where there's no discernible similarity between species, says Moran. This means many sequences will not line up when you compare genomes between species.

And the further away two species are on the evolutionary tree, the greater the difference.

"If we compare really closely related species, like a human and chimpanzee, we can still see the similarity between these rapidly changing sequences. If you move further away to the more distantly related pig, so many changes in the DNA will have occurred that it is no longer possible to recognise that the sequences were ever similar.

"Depending upon what it is that you are comparing you can say 'Yes, there's a very high degree of similarity, for example between a human and a pig protein coding sequence', but if you compare rapidly evolving non-coding sequences from a similar location in the genome, you may not be able to recognise any similarity at all. This means that blanket comparisons of all DNA sequences between species are not very meaningful."

One area where comparison of genome sequences isn't all that relevant, says Moran, is the emerging science of transplanting organs and tissues from pigs to humans.

"[The success of pig-human transplants] has very little to do with whether there's a two per cent or 20 per cent difference in the genome sequence — if those numbers actually meant anything anyway — the main barrier is caused by just one gene," says Moran.

That gene is called galactose-alpha-1,3,galactotransferase — gal-transferase for short . All mammals except humans and higher apes have a working version of gal-transferase, which coats cells with an antigen (a molecule that our immune system reacts to). This means if pig tissue is transplanted into humans our immune system will mount a drastic rejection response as our bodies detect the antigen and attack it.

Scientists have come up with a solution to stop tissue rejection: genetically modifying the pigs by eliminating the gal-transferase gene. A few more human genes are also added to the pigs to make the pig tissue even more acceptable to our immune system.

So some pigs and humans are now even more alike.

Chris Moran is a professor of animal genetics at the University of Sydney's Faculty of Veterinary Science. He was interviewed by Genelle Weule.