In this chapter, we focused on extranuclear inheritance and how traits can be determined by genetic information contained in mitochondria and chloroplasts, and we discussed how expression of maternal genotypes can affect the phenotype of an organism. At the same time, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions?

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

Key Concepts

Extranuclear Inheritance

Extranuclear inheritance refers to the transmission of genetic material that occurs outside the nucleus, primarily involving mitochondria and chloroplasts. Unlike nuclear DNA, which follows Mendelian inheritance patterns, extranuclear DNA is often inherited maternally. This means that traits determined by these organelles can be passed down from the mother to offspring, influencing characteristics such as colony size in yeast.

Petite Mutations

Petite mutations are specific genetic alterations found in yeast that result in a reduced ability to produce energy through oxidative phosphorylation, leading to smaller colony sizes. Researchers identified three categories of these mutations, which differ in their effects on mitochondrial function. The observation that these mutations could affect colony size suggested that the genetic information responsible for this trait was located in the mitochondria, supporting the concept of extranuclear inheritance.

Maternal Effect

The maternal effect is a phenomenon where the genotype of the mother directly influences the phenotype of her offspring, regardless of the offspring's own genotype. In the context of extranuclear inheritance, maternal genotypes can determine the characteristics of organelles like mitochondria, which are passed to the offspring. This effect is crucial for understanding how traits linked to mitochondrial DNA, such as those seen in petite mutations, manifest in the phenotype of the organism.