If the mother and grandmother of a given family both exhibit a half dozen phenotypic effects of a ...

Phenotypes are directly dependent on the number or percentage of mutated mitochondria per cell. Thus, cells that inherit more of the mitochondria with the mutation will show a more severe phenotype than those with enough normal mitochondria to maintain the wild-type function of the gene. If, during embryonic development, the cells that gave rise to a particular organ or tissue are derived from a cell with more of the mutant mitochondria, that individual will inherit the phenotype associated with that particular tissue defect. Similarly, if the cell that gave rise to a particular tissue or organ had mostly normal mitochondria, that individual may be phenotypically normal with regard to that tissue or organ.