Inbreeding depression is reduced fitness in offspring of related parents: slower growth, smaller plants, poorer seed set, weak germination, deformities or more susceptibility to stress. The main cause is that inbreeding makes harmful recessive alleles homozygous, so their effects show; loss of heterozygote advantage adds to it (Charlesworth and Willis 2009).
In practice
Cannabis is naturally outcrossing, so it carries a load of hidden recessives and can suffer when inbred hard. Breeders see it after repeated selfing (S1 on S1) or several sib generations with small populations. Signs: falling germination rates, runts, twisted leaves, lower yields, and in some lines unstable sex expression.
The defence is selection and population size. Cull hard each generation, keep many parents, and outcross when a line weakens. A good IBL is one where the breeder purged the defects along the way rather than inbreeding past them.
Don't confuse with
- Clone decline: clones are not inbred and do not inbreed. A cut that weakens over years almost always has a pathogen (such as hop latent viroid) or a care problem.
- Genetic drift: random loss of alleles in small populations; it often accompanies inbreeding but is a separate process.
- Heterosis: the opposite effect, from crossing unrelated lines.
Germination is the stage where a viable seed absorbs water, splits its shell and pushes out a root (radicle), usually within 1-7 days at 20-30 °C.Inbreeding is mating between related plants, which raises homozygosity; an inbred line is the resulting population, measured by the inbreeding coefficient F.A recessive allele only shows its effect when a plant carries two copies of it; in a single copy it is masked by a dominant allele and carried silently.An outcross is a cross to an unrelated plant from outside the line, used to bring in new traits or vigour, or to rescue a line suffering inbreeding depression.S1 is seed from a plant pollinated with its own pollen; every S1 seed is female and a new genotype, not a copy of the parent.Genetic drift is random change in allele frequencies between generations of a seed population, strongest in small populations; clones do not drift this way.HLVd, a small infectious RNA that spreads through sap and tools, causing stunting, brittle stems, small loose buds and reduced resin in cannabis; called dudding.An allele is one version of a gene; a diploid cannabis plant carries two alleles at each locus, one from each parent, which may be the same or different.Selfing is pollinating a plant with its own pollen, in cannabis usually by reversing part of a female to make pollen; it is the most intense form of inbreeding.Heterosis (hybrid vigour) is the tendency of offspring from genetically different parents to outgrow or outperform both parents in traits such as vigour and yield.A genetic bottleneck is a sharp drop in the number of breeding plants, after which the population carries only a fraction of its former genetic diversity.
