Filial Generation – F1, F2 and F3 in a Breeding Sequence

Filial generation is nothing more than a way of counting. F1 is the first daughter generation from two parent plants, F2 comes from their offspring, F3 from the same operation one step further on. The number says nothing about quality and nothing about uniformity; it says only how many crossing steps have passed since the founding pair. Reading it as a mark of quality reads in something that is not there.

Why the first generation is said to be uniform

The image of a uniform F1 comes from one particular case: two lines that are each true-breeding in themselves but differ in the traits under observation. True-breeding means a plant carries the same variant twice at the places in question and therefore passes them on unchanged. Because both parents do so, every offspring plant can only receive the same combination, and the cross turns out the same every time.

In maize, the road to such lines runs through several generations in which each plant is crossed with itself. The resulting inbred lines differ from one another, yet each is uniform in itself.

For species that pollinate across individuals rather than themselves, the opposite holds, explicitly. There, selection begins in the first daughter generation already, because the wanted combinations can appear there – and because the offspring will not look alike in any case.

What that means for cannabis

An F1 from two selected individual plants is not an F1 in the textbook sense, because the two parents are not true-breeding but carry two different variants at many places. How far even repeatedly inbred lines still are from that has been measured and is covered in the article on homozygosity. Genuine hybrid seed only arises from crossing two unrelated true-breeding lines, and those take roughly six generations of close mating.

The term is used loosely on the market all the same. Reading “F1” on a packet tells you that a cross was made, not that the offspring will come out alike.

What the second generation shows

The F2 is the generation in which traits recombine independently of one another, and therefore the first in which there is anything to select. For traits where one variant masks the other, the share can be calculated: with n such traits, (3/4)^n of the plants show the wanted appearance and only (1/4)^n carry it true-breeding – across three traits about 1.6 %, which in practice means one or two usable plants among a hundred seedlings. The F3 fixes what the F2 turned up: outliers beyond the parental range, for instance, become heritable from the second daughter generation onward.

Why the step to F2 is ambiguous in cannabis

Cannabis normally carries male and female flowers on separate plants. From a single selected plant, the step to F2 therefore runs only through a chemically induced sex reversion, in which a female plant forms fertile male flowers. Mate two F1 siblings instead and the same generation arises without any chemical intervention, though from two different genetic makeups rather than one.

Both routes are called F2 and do not deliver the same thing. Glossing over the difference means using one word for two things.

Sources