How did the discovery of three categories of petite mutations in yeast lead researchers to postulate extranuclear inheritance of colony size?

Answer;

Whereas segregational petites exhibited Mendelian inheritance, both neutral and suppressive petites followed non-Mendelian patterns that were consistent with the involvement of an extranuclear agent

Explanation; Mutations that yield defective mitochondria are expected to make cells grow much more slowly. These mutants were called petites to describe their small colonies compared to large wild type colonies. Petite mutants could not grow when cells had an energy source requiring only metabolic activity of mitochondria - needed sugar as well which is part of glycolytic pathway. Segregational petites, segregated in mendelian manner during meiosis. mutations cause defects in genes in cell nucleus encode proteins necessary for mitochondrial function. Vegetative petite mutants do not segregate in mendelian manner; two types: neutral and suppressive; carry mutations in mitochondrial genome itself; when two yeast cells are mated, daughter cells inherit mitochondria from both parents. Neutral petites lack most of their mitochondrial DNA; when mated with wildtype, the wildtype give their mitochondria so all cells display a normal phenotype.