Which genetic mechanism explains how variants of viral genomes differentiate and cause pandemics?

• A. Mutation
• B. Genetic drift
• C. Genetic shift
• D. Recombination

Answer: (C)

Genetic shift refers to a major genetic change that happens when different strains of a segmented virus infect the same cell and swap whole genome segments. In influenza, this reassortment can produce a new subtype with a novel combination of surface proteins, so most people have little immunity. That sudden, big change allows the virus to spread widely, causing pandemics. By contrast, mutation causes small, gradual changes over time (antigenic drift) and leads to seasonal flu rather than pandemics. Recombination can mix genetic material in some viruses, but the large-scale emergence of pandemic strains in influenza is best explained by this rapid reassortment of genome segments—genetic shift.