Slow-growing yeast cells called neutral petites lack normal activity of the respiratory enzyme cytochrome oxidase associated with the mitochrondria. Petites can be maintained indefinitely in...

Slow-growing yeast cells called neutral petites lack normal activity of the respiratory enzyme cytochrome oxidase associated with the mitochrondria. Petites can be maintained indefinitely in vegetative cultures through budding, but can sporulate only if crossed to wild type. When a haploid neutral petite cell fuses with a haploid wild-type cell of opposite mating type, a fertile wild-type diploid cell is produced. Under appropriate conditions, the diploid cell reproduces sexually (sporulates). The four ascospores of the ascus (Fig. 6-4) germinate into cells with a 1 : 1 mating type ratio (as expected for nuclear genes), but they are all wild type. The petite trait never appears again, even after repeated backcrossings of both mating types to petite. The mitochondrial factors for petite are able to perpetuate themselves vegetatively, but are “swamped,” lost, or permanently altered in the presence of wild-type factors. Neutral petite behaves the same in reciprocal crosses regardless of mating type. Assume that a neutral petite yeast has the chromosomal genes for normally functioning mitochondria, but has structurally defective mitochondria. Another kind of yeast is known, called segregational petite, which has structurally normal mitochondria that cannot function because of inhibition due to a recessive mutant chromosomal gene. What results would be expected among the sexual progeny when the neutral petite crosses with the segregational petite?