Advanced Placement (AP) Biology Practice Exam

What is the significance of understanding extranuclear genes in genetics?

1. They are irrelevant to inheritance

2. They help explain maternal inheritance patterns

3. They provide insight into chromosomal disorders

4. They are exclusively related to mitochondrial diseases

The correct answer is: They help explain maternal inheritance patterns

Understanding extranuclear genes is significant primarily because they play a critical role in explaining maternal inheritance patterns. Extranuclear genes, which are typically found in organelles such as mitochondria and chloroplasts, are inherited strictly through the maternal line. This is due to the fact that, during fertilization, the egg contributes the cytoplasm and its associated organelles to the zygote, while the sperm contributes primarily nuclear genetic material. This maternal inheritance pattern has important implications for genetic traits that are linked to these extranuclear genes. For instance, mitochondrial DNA contains genes crucial for energy production, and mutations in these genes can lead to disorders that are passed from mother to offspring regardless of the paternal genetic makeup. While the other options touch on aspects of genetics, they do not adequately capture the importance of extranuclear genes in inheritance patterns. For example, extranuclear genes are indeed linked to diseases, but they are not exclusively related to mitochondrial diseases, as option D suggests. Additionally, while chromosomal disorders are an important aspect of genetics, they primarily involve nuclear DNA rather than extranuclear genes. Thus, the best reason to understand extranuclear genes is their role in elucidating patterns of maternal inheritance.