For the human population to maintain a constant size from generation to 
generation, an increase in fertility must compensate for the reduction in the 
mean fitness of the population caused, among others, by deleterious mutations. 
The required increase in fertility due to this mutational load depends on the 
number of sites in the genome that are functional, the mutation rate, and the 
fraction of deleterious mutations among all mutations in functional regions. 
These dependencies and the fact that there exists a maximum tolerable 
replacement level fertility can be used to put an upper limit on the fraction of 
the human genome that can be functional. Mutational load considerations lead to 
the conclusion that the functional fraction within the human genome cannot 
exceed 25%, and is probably considerably lower.