The mrp20-A105E mutation occurred spontaneously, increasing phenotypic variance in the BYx3S cross. a) A spontaneous mutation in a BY/3S diploid gave rise to a BYx3S segregant population in which mrp20-A105E segregated. b) The mrp20-A105E segregants exhibited increased phenotypic variance and a bimodal distribution of phenotypes. Throughout the study, blue and orange are used to denote BY and 3S genetic material, respectively. All growth data presented in the paper are measurements of colonies on agar plates containing rich medium with ethanol as the carbon source.