Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

depression” upon repeated self-breeding or inbreeding.

A male plant is fertilizing a female.

Inbreeding consists of crossing a group, family, or variety of plants with themselves.
    Inbreeding depression is a reduction in vigor (or any other character) due to prolonged inbreeding. This can manifest as a reduction in potency or a decrease in yield or rate of growth. Progress of depression is dependent, in part, on the breeding system of the crop. Earlier, when we discussed dioecy, we said cannabis is an outcrossing or cross-pollinating species. Cross-pollinated crops usually exhibit a higher degree of inbreeding depression when “selfed,” or inbred, than do selfing crops. For example, tomato (an inbreeding or selfing species) can be selfed for 20 generations with no apparent loss in vigor or yield, whereas some experiments have shown that the yield of corn per acre is decreased quite dramatically wheninbred for 20 generations.
    In cross-pollinated crops, deleterious genes remain hidden within populations, and the negative attributes of these recessive traits can be revealed or unmasked via continual inbreeding. Inbreeding depression can be apparent in S1 populations after a single generation of self-fertilization. When breeding cannabis using small populations, as is often the case with continual 1:1 mating schemes, inbreeding depression typically becomes apparent within three to six generations. To deal with this problem, breeders often maintain separate parallel breeding lines, each of which are selected for similar or identical sets of traits. After generations of inbreeding, when each of the inbred lines, or selfed populations, begin to show inbreeding depression, they are hybridized or outcrossed to each other to restore vigor and eliminate inbreeding depression while preserving the genetic stability of the traits under selection.
    The vast majority of texts written to date on the subject of breeding cannabis have espoused 1:1 mating strategies, much to the detriment and health of cannabis germplasm. Sadly, this is the preferred breeding scheme used today by the majority of commercial seed banks. These breeders don’t realize that cannabis is naturally an out-crossing or cross-pollinating species and existed in wild breeding populations of hundreds if not thousands of individuals. Within these many individuals lies a wide range of versions of different genes. When we select only one or two plants from this vast array as our breeding population, we drastically reduce the genetic variability found in the original population (a genetic bottleneck). This variability is lost from the populations, and unavailable to future generations.

This pyramid shows the evolution of breeding cannabis.
Outbreeding
    Outbreeding is the process of crossing or hybridizing plants or groups of plants with other plants to which there is no, or only a distant, relation. Any time a breeder is hybridizing using plants that reside outside of the family, group, or variety, hybrid seed is produced. For example, an F1 hybrid seed is the first generation offspringresulting from a cross of two distinct true-breeding plants or populations. Each of the parent populations were hybridized (outcrossed to each other) to produce the new generation, which is now comprised of genetics from both parental populations. Outcrossing results in the introduction of new and different genetic material to each of the respective pools.
    Filial Breeding - A type of breeding system where siblings of the same progeny lot and generation are intermated to produce new generations. The first hybrid generation of two distinct true-breeding lines is denoted the F1 generation (F, filial). If two F1 siblings are bred, or the F1 population is allowed to be open pollinated, the resulting generation is labeled F2.
    Mating siblings chosen from the F2, results in the F3 population. F4, F5, F6 generations, etc., are obtained in the same manner, by crossing plants of the same generation and progeny lot. Note that as long as any number of siblings of a generation (F[n]) are mated, the resulting generations is denoted (F[n+1]).
    Filial inbreeding with selection for specific traits is the most common method for establishing a pure or a true-breeding population, when breeding cross-pollinated species such as cannabis.
    Backcross Breeding - A type of breeding that involves repeated crossing of progeny with one of the original parental genotypes; cannabis breeders most often cross