In the discussion, 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 findings demonstrate a maternal effect as the basis of a mode of inheritance?

Verified step by step guidance

1. Understand the concept of maternal effect: Maternal effect refers to a situation where the phenotype of an organism is determined not by its own genotype, but by the genotype of its mother. This occurs because the mother provides mRNA, proteins, or other factors to the egg during oogenesis, which influence early development.
2. Identify examples of maternal effect: One classic example is the inheritance of shell coiling direction in snails (e.g., *Lymnaea*). The direction of coiling (dextral or sinistral) is determined by the genotype of the mother, regardless of the offspring's genotype.
3. Differentiate maternal effect from extranuclear inheritance: Maternal effect involves nuclear genes in the mother that influence the offspring's phenotype, while extranuclear inheritance involves genetic material in organelles like mitochondria or chloroplasts, which are inherited maternally.
4. Examine experimental evidence: Studies demonstrating maternal effect often involve reciprocal crosses. For example, in the snail shell coiling example, crossing a dextral mother with a sinistral father produces dextral offspring, while reversing the cross produces the same result, showing that the mother's genotype determines the phenotype.
5. Summarize findings: The key findings that demonstrate maternal effect include the observation that the offspring's phenotype is determined by the maternal genotype, not the offspring's own genotype, and that this effect is consistent across reciprocal crosses.

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. Unlike nuclear DNA, which is inherited from both parents, mitochondrial DNA is typically inherited maternally. This type of inheritance can influence traits and phenotypes, particularly in organisms where maternal contributions are significant.

Maternal Effect

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. This occurs because the mother provides cytoplasmic factors, such as RNA and proteins, during oocyte development. As a result, the traits expressed in the offspring can be a direct reflection of the maternal genotype, even if the offspring carry different alleles.

Phenotypic Expression

Phenotypic expression refers to the observable characteristics or traits of an organism, which result from the interaction of its genotype with the environment. In the context of maternal effects, the phenotypic traits of offspring can be influenced by maternal genotypes and the substances provided during development. Understanding how these traits manifest helps in studying inheritance patterns and the role of maternal contributions in shaping phenotypes.