Inbreeding

Marijuana Horticulture

by Jorge Cervantes

Inbreeding is nothing more than crossing a group, family, or variety of plants within themselves with no additions of genetic material from an outside or unrelated population. The most severe form of inbreeding is the self-cross, in which only one individual’s genetic material forms the basis of subsequent generations. 1:1 hybrid populations are only slightly less narrow, derived from the genetic material of 2 individuals. Such tight or narrow breeding populations lead to a condition called “inbreeding depression” upon repeated self-breeding of inbreeding.

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 when inbred 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 of identical sets of traits. After generations of inbreeding, when each of the inbred lines, or selfed populations, begin t 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 t 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.