text: Phenotypic threshold effect is when there is a certain amount of wild-type mtDNA present in the mitochondrion which is able to balance out the number of mutated mtDNA. As a result, the phenotype is normal. However, if the number of wild-type mtDNA decreases and the number of mutant mtDNA increases, resulting in an imbalance between the two, the threshold level has been altered which causes complications. This occurs because the wild-type mtDNA present are able to keep the electron transport chain and ATP synthesis functioning despite there being a few number of them present. They are able to counterbalance the mutated mtDNA, however, when the number drops below threshold level the mutant mtDNA take over.