In:
Microbiological Reviews

The expression "vertical transmission" refers to the direction of transmission when family trees are drawn on paper. They are represented as lines beginning from a trunk (the ancestors) at the top of the page, the descendants occupying the proliferating branches further down the page. It was because such diagrams looked like the branching digits of a bird that the word pedigree (pied de grue = foot of the crane) came into our language. Relationships that are in this sense vertical have been the classical concem of the geneticist, and the hereditary elements are transmitted down the page, and down from generation to generation. The vertical axis is time, and a given hereditary unit (gene) can in this way be charted over the course of many generations. This is in contrast to the transmission of materials between individuals who are living at the same time, this being represented as a more or less horizontal line on the diagram. Horizontal transmission can take place between related or between unrelated individuals.

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