There’s been a bit of a tussle in the genetics world concerning how much of our DNA is functional, and how much of it is just junk. Heck, there’s even a disagreement over the meaning of ‘junk.’ But it had not occurred to me to wonder how to estimate these numbers – and this has some challenge given the human genome is made up of 3 billion base pairs, comprising roughly 30,000 genes. Add in the fact that many genes act in concert with each other, and the problem is compounded again. Michael Le Page in NewScientist (22 July 2017) describes one approach – assume a value and then look for conclusions not congruent with reality:
… if most of our DNA is functional, most mutations would fall in important sequences and be bad for us. But if most of our DNA is junk, the majority of mutations would have no effect. [Professor Dan] Graur’s team has now calculated how many children a couple would need to conceive for evolution to stop us accumulating too many bad mutations in different cases.
If the entire genome has a function, couples would need to have around 100 million children, and almost all would have to die, they found. Even if just a quarter of the genome is functional, each couple would still need to have nearly four children on average, with only two surviving to adulthood.
Taking into account estimates of the mutation rate and average prehistorical reproduction rate, Graur’s team calculated that only around 8 to 14 per cent of our DNA is likely to have a function (Genome Biology and Evolution, doi.org/b9q3). This ties in nicely with a 2014 study that compared our genome with other species and concluded that around 8 per cent of it is functional.
“We are walking around with a genome where only 1 in 10 bases actually matters,” says Chris Ponting of the University of Edinburgh, UK, who was part of the 2014 study.
The challenge for those who still think most non-coding DNA is vital is to explain why an onion needs five times as much of it as we do, says Ryan Gregory of the University of Guelph in Canada.
So long as you understand that evolution is a messy, imprecise business, this actually makes sense. Add in the information that our genomes can actually be used by viruses, which have their own goals, and it’s no surprise that big chunks of our genome are foreign, deactivated, non-functional, and only a bit of it is functional.
