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?

What key observations in crosses between dextrally and sinistrally coiled snails support the explanation that this phenotype is the result of maternal-effect inheritance?

Key Concepts

Extranuclear Inheritance

Extranuclear inheritance refers to the transmission of genetic material that occurs outside the nucleus, primarily through organelles like mitochondria and chloroplasts. This type of inheritance is distinct from Mendelian inheritance, as it involves genes that are not located on the chromosomes within the nucleus. In many organisms, traits determined by extranuclear genes can be passed down maternally, influencing the phenotype of offspring based on the genotype of the mother.

Maternal Effect Inheritance

Maternal effect inheritance occurs when the phenotype of an offspring is determined not just by its own genotype but also by the genotype of its mother. This is particularly evident in cases where maternal factors, such as RNA or proteins deposited in the egg, influence early development. In the context of the snail crosses, the maternal genotype can dictate the coiling direction of the offspring, regardless of the offspring's own genetic makeup.

Phenotypic Expression in Snails

The phenotypic expression in dextrally and sinistrally coiled snails serves as a classic example of maternal effect inheritance. In these snails, the direction of coiling is determined by the maternal genotype, which affects the development of the offspring. Observations from crosses between these two types of snails reveal that the coiling direction of the offspring aligns with the maternal phenotype, supporting the idea that maternal genotypes play a crucial role in determining certain traits.