Horizontal gene transfer describes gene transfer between what?

• A. Gene transfer via viruses only
• B. Gene transfer between unrelated species
• C. Gene transfer within a single species
• D. Gene transfer from parent to offspring

Answer: (B)

Horizontal gene transfer is the movement of genetic material between organisms that are not parent and offspring. This most often occurs between different species, allowing recipients to gain new traits quickly, such as antibiotic resistance moving from one bacterial species to another via plasmids or phages. It is distinct from vertical transfer, which is from parent to offspring, and it is not limited to viruses—there are several mechanisms, including transformation, conjugation, and transduction, that can mediate transfer. Therefore, the description that fits best is transfer between unrelated species.