F factor
 A bacterial episome that confers the ability to function as a genetic donor in conjugation; the fertility factor in bacteria.

Fl The first filial generation in a cross between any two parents; the first generation of descent from a given mating
F2The second filial generation, produced by crossing two members of the Fl or by self-pollinating the Fl. The grandchildren of a given mating.

Factorial mating
A mating scheme in which each male parent is mated with each female parent. Made possible in animals by means of in vitro embryo production (q.v.). Such a mating scheme substantially reduces the rate of inbreeding in a selection programme

Facultative
A group of microorganisms which prefer or preferentially use molecular oxygen when available, but are capable of suing other pathways for energy and synthesis if molecular oxygen is not available.

False fruit
See pseudocarp
False-negative
A negative assay result that should have been positive.

False-positive
A positive assay result that should have been negative

Fascicle
See vascular bundle.

Feedback inhibition
The process by which the accumulated end product of a biochemical pathway stops synthesis of that product. A late metabolite of a synthetic pathway regulates synthesis at an earlier step of the pathway. cf end-product inhibition.

Fermenter
See bioreactor.