How much of the human genome, our genetic blueprint, actually makes us who we are? New work by an evolutionary biologist at the University of Houston suggests that only up to 25% of the human genome is functional. The other 75% are so-called “junk DNA” - useless sequences that don’t play a role in the important chemical reactions inside us. This conclusion goes sharply against the estimate of 80% functionality proposed by the ENCODE project, an international public research consortium that has led the way in human genome exploration.

Dan Graur, professor of biology and biochemistry, calculated that about 10 to 15 percent of the genome is actually functional, with the upper limit of 25 percent. 

His reasoning stems from looking at how mutations affect a population’s DNA. Graur’s mathematical model allowed him to calculate the “mutational load” - the total genetic load of a population that results from the accumulation of bad or deleterious mutations. At some point the load can become too much and the population would go extinct.  

Graur’s work related how reproductive success, the ability of a species to replenish itself, was decreased by the deleterious mutations. Over time, humans would have to reproduce at an impossible high rate to keep up with the mutations, Graur concluded.

The professor explained why he finds the 80% functionality of the genome proposed by the ENCODE scientists as unrealistic:

“For 80 percent of the human genome to be functional, each couple in the world would have to beget on average 15 children and all but two would have to die or fail to reproduce,” writes Graur. “If we use the upper bound for the deleterious mutation rate (2 × 10−8 mutations per nucleotide per generation), then … the number of children that each couple would have to have to maintain a constant population size would exceed the number of stars in the visible universe by ten orders of magnitude.”

This is not the first time Graur fought against the 80% claim. In a 2014 interview with Science magazine, Graur even claimed its proponents are essentially pitching the idea of “intelligent design”. To Graur, asserting 80% usability implies that most of the genome exists to serve a purpose. Instead, he believes that “everything is shaped by evolution,” a slow process that weeds out useless features through genetic mutations - the drivers of evolution. This process also accumulates a lot of junk in the human genome.

Why is it important to know that only a quarter of the human genome may have functionality? Graur believes his work can shift the focus in the field of human genomics to what is useful from a medical standpoint: 

“We need to know the functional fraction of the human genome in order to focus biomedical research on the parts that can be used to prevent and cure disease,” said Graur. “There is no need to sequence everything under the sun. We need only to sequence the sections we know are functional.” 

You can read the study here in Genome Biology and Evolution.