Genes pass only from parents to their offspring.

The familiar picture of inheritance is vertical. Parents pass genes to offspring, and over generations lineages branch apart.

Microbes revealed that DNA can sometimes move sideways as well.

Bacteria have several routes for this. In transformation, a cell takes up loose DNA from its surroundings. In conjugation, DNA is transferred directly from one bacterial cell to another. In transduction, a virus that infects bacteria carries DNA between cells.

These processes are forms of horizontal gene transfer because the DNA moves between organisms without parent-to-offspring reproduction.

The medical importance is easy to see in antibiotic resistance. A useful resistance gene can move into a new bacterial population instead of waiting for that population to evolve the same protection independently.

Whole-genome comparisons later showed that horizontal transfers have also shaped microbial evolution over very long periods. Some genes have histories that do not match the simple branching history of the organisms carrying them.

Vertical inheritance still dominates for most genes in animals and plants, and family trees remain extremely useful. Horizontal gene transfer adds another route, especially in microbes; it does not replace ordinary inheritance.