A genetic bottleneck happens when a population is reduced to very few breeding individuals. Only the alleles carried by the survivors pass on; rare alleles are often lost, and the population afterwards is more inbred and less variable, even if it later grows large again.
In practice
Cannabis has passed through many bottlenecks. Landraces carried out of source regions in a single seed packet; a whole line rebuilt from three plants after a raid or crop loss; a modern cultivar group descended mostly from a handful of famous clones. Each step narrows what breeders have to work with.
Breeders create bottlenecks themselves by making each generation from one female and one male. The cure is prevention: use many parents, keep backup seed, and preserve material from different sources.
Don't confuse with
- Founder effect: a special case, when a new population is started by a few individuals.
- Genetic drift: random change in allele frequency; it is strongest during and after a bottleneck.
- Selection: keeping plants on purpose for traits; a bottleneck is a loss in numbers, chosen or not.
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.A landrace is a traditional, locally adapted cannabis population kept by farmers in one region over many generations, with its own type and plenty of internal variation.Pest and disease prevention by changing how the crop is grown, such as humidity, airflow, spacing, watering and cultivar choice, so pests and pathogens struggle to establish.The founder effect is the loss of genetic diversity when a new population is started from a few individuals, so it carries only a sample of the source population's alleles.Genetic drift is random change in allele frequencies between generations of a seed population, strongest in small populations; clones do not drift this way.The gene pool is the total set of alleles present in a population, cultivar group or species; a narrow gene pool limits what breeders can select from.Inbreeding depression is the loss of vigour, fertility, yield or survival that follows mating between relatives, mainly because harmful recessive alleles become homozygous.Germplasm is living genetic material (seeds, pollen, tissue or plants) kept for breeding, research or conservation, representing the genetic diversity of a crop.
