You have isolated two petite mutants, pet1 and pet2, in Saccharomyces cerevisiae. When pet1 is mated with wild-type yeast, the haploid products following meiosis segregate 2:2 (wild type : petite). In contrast, when pet2 is mated with wild type, all haploid products following meiosis are wild type. To what class of petite mutations does each of these petite mutants belong? What types of progeny do you expect from a pet1 × pet2 mating?

Key Concepts

Mitochondrial Genetics

Mitochondrial genetics refers to the study of genes located in the mitochondria, which are inherited maternally. In yeast, petite mutations often arise from defects in mitochondrial DNA, affecting cellular respiration. Understanding this concept is crucial for distinguishing between nuclear and mitochondrial mutations, especially in the context of yeast mutants like pet1 and pet2.

Meiosis and Segregation

Meiosis is a type of cell division that reduces the chromosome number by half, resulting in four haploid cells. The segregation of alleles during meiosis can lead to different phenotypic ratios in the offspring. In the case of pet1, the 2:2 segregation indicates a nuclear mutation, while the uniform wild-type offspring from pet2 suggests a mitochondrial mutation, as mitochondrial traits do not segregate in the same manner.

Complementation Testing

Complementation testing is a genetic technique used to determine whether two mutations are in the same gene or in different genes. By mating pet1 and pet2, we can assess whether the petite phenotypes can complement each other, which would indicate they are mutations in different genes. This is essential for predicting the progeny types from a pet1 × pet2 cross, as it helps clarify the genetic basis of the petite phenotype.