In a study published in the online science journal Genome Biology and Evolution, Professor Dan Graur of the University of Houston claims that only 10-15 percent (with 25 percent being the upper limit) of our genes are functional, as opposed to 80 percent – a figure proposed by scientists working on the ENCODE project.
To arrive at the new rate of functional DNA, Graur relied on genome size, the deleterious mutation rate – that is, the rate at which harmful mutations occur – the replacement fertility rate, and historical data documenting human population levels.
With all the necessary data on hand, Graur developed a model that links the decrease in reproductive success caused by harmful mutations, called ‘mutational load’, and the proportion of functional and non-functional DNA.
The model proposes that harmful mutations can only have meaning (i.e., be described as ‘harmful’) only in functional portions of the genome, as mutations in non-functional DNA (i.e., a part of the genome that doesn’t have any function that arose through and is maintained by natural selection) are neutral because a non-function can be neither damaged, nor improved.
According to Graur’s calculations, in order to maintain a relatively stable population size and offset the negative impact of deleterious mutations, every couple living before the 19th century (at which time fertility rates began exceeding replacement levels) had to beget slightly more than two children.
Given that, and supposing that 80 percent of the genome was functional, sustaining a population would require almost astronomically high birth rates even at the low end of the estimated rate of harmful mutations.
“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,” said Graur. “If we use the upper bound for the deleterious mutation rate (2 x 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 galaxy by ten orders of magnitude”.
The new findings, Graur hopes, will allow scientists to focus on sequencing only the most relevant parts of the genome (e.g., those related to disease) rather than working tirelessly to “sequence everything under the sun”.