FIGURE 12-9 Cytoplasmic inheritance of an mtDNA petite mutation in yeast. Petite-strain mitochondria are defective in oxidative phosphorylation owing to a deletion in mtDNA. (a) Haploid yeast cells fuse to produce a diploid cell that undergoes meiosis, during which random segregation of parental chromosomes and mitochondria containing mtDNA occurs. Note that alleles for genes in nuclear DNA (represented by large and small nuclear chromosomes colored red and blue) segregate 2:2 during meiosis (see Figure 6-5). In contrast, since yeast normally contain some 50 mtDNA molecules per cell, most products of meiosis contain both normal and petite mtDNAs and are capable of respiration. (b) As these haploid cells grow and divide mitotically, the cytoplasm (including the mitochondria) is randomly distributed to the daughter cells. Occasionally, a cell is generated that contains only petite mtDNA and yields a petite colony. Thus formation of such petite cells is independent of any nuclear genetic marker.