Advanced Placement (AP) Biology Practice Exam

How are extranuclear genes inherited?

1. Paternally from the father

2. Maternal inheritance from the mother

3. Equally from both parents

4. Randomly without a pattern

The correct answer is: Maternal inheritance from the mother

Extranuclear genes, found in organelles such as mitochondria and chloroplasts, are primarily inherited through the maternal lineage. This is because, during fertilization, the sperm contributes very little to the embryo's cytoplasm, which heavily contains mitochondria and other organelles. As a result, the majority of the mitochondrial DNA present in the offspring comes from the egg cell. The maternal inheritance pattern emphasizes that the traits associated with these extranuclear genes are passed down from mothers to their offspring, leading to similar mitochondrial characteristics within a lineage. This phenomenon is crucial for understanding various genetic disorders that may arise from mutations in mitochondrial DNA, impacting energy production and cellular function. While it is essential to recognize that paternal contributions are minimal in terms of extranuclear DNA inheritance, other inheritance patterns apply to nuclear genes, which can be influenced by both parents. This distinction clarifies why maternal inheritance is the characteristic mode for these specific types of genes.