What is the simplest explanation for why patienas have been identified with only one copy of the phosphofructokinase-1 gene (heterozygous), but no patients have been identified that lack both copies of the phosphofructokinase-1 gene (homozygous)? Patients lacking both copies of the phosphofructokinase-1 genes will be found once DNA sequencing technology can sequence whole genomes. Phosphofructokinase-1 is a required enzyme for carbohydrate metabolism in all living cells, complete loss of this enzyme would be lethal. There are 6 phosphofructokinase-1 paralogous genes in humans and it is impossible to lack both type 1 copies when there are also types 4,5 , and 6. Phosphofructokinase 1 is needed for nitrogen metabolism and there are no enzymes to replace this function, so cells die from ammonia toxicity