Question

The specification of the anterior–posterior axis in Drosophila embryos is initially controlled by various gene products that are synthesized and stored in the mature egg following oogenesis. Mutations in these genes result in abnormalities of the axis during embryogenesis, illustrating maternal effect. How do such mutations vary from those involved in organelle heredity that illustrate extranuclear inheritance? Devise a set of parallel crosses and expected outcomes involving mutant genes that contrast maternal effect and organelle heredity.

Accepted Answer

Step 1: Understand the difference between maternal effect genes and organelle heredity. Maternal effect genes are nuclear genes whose products (mRNA or proteins) are deposited by the mother into the egg and influence early embryonic development. Mutations in these genes affect the offspring's phenotype based on the mother's genotype, regardless of the offspring's own genotype. In contrast, organelle heredity involves genes located in extranuclear DNA (such as mitochondrial or chloroplast DNA), and inheritance is typically uniparental (often maternal), affecting traits passed through cytoplasmic organelles. Step 2: Design a cross to illustrate maternal effect inheritance. For example, cross a homozygous mutant female (for a maternal effect gene) with a wild-type male. Since the mother is mutant, all offspring will show the mutant phenotype regardless of their own genotype. Then, cross a heterozygous or wild-type female with a mutant male; offspring will show the wild-type phenotype because the mother's genotype determines the phenotype. Step 3: Design a parallel cross to illustrate organelle heredity. Cross a female carrying mutant mitochondria (or other organelle mutation) with a wild-type male. Because organelle DNA is inherited maternally, all offspring will inherit the mutant organelle phenotype. Conversely, crossing a wild-type female with a mutant male will produce offspring with wild-type phenotype, as organelles are not inherited from the father. Step 4: Summarize expected outcomes in a table or list contrasting the two inheritance patterns: for maternal effect, offspring phenotype depends solely on the mother's nuclear genotype; for organelle heredity, offspring phenotype depends on the mother's cytoplasmic genotype (organelle DNA). Step 5: Conclude by explaining how these crosses demonstrate the fundamental difference: maternal effect involves nuclear genes expressed in the mother affecting early development, while organelle heredity involves extranuclear genes inherited through cytoplasmic organelles, both leading to non-Mendelian inheritance patterns but via distinct mechanisms.

Key Concepts

Maternal Effect Genes

Organelle Heredity and Extranuclear Inheritance

Genetic Crosses to Distinguish Maternal Effect and Organelle Inheritance