Introgression is the transfer of genetic material from one population into another, through a cross followed by repeated backcrossing to the receiving population. The result is a population that is mostly the recipient but carries segments of the donor's DNA.
In practice
Breeders introgress traits on purpose: move the autoflowering allele, a disease resistance or a chemotype into an elite line by crossing then backcrossing several times. The CBD-producing alleles in many drug-type cannabis lines appear to come from hemp introgressed into marijuana (Grassa et al. 2021).
Introgression also happens unintentionally. When imported hybrid seed is grown near traditional fields, its pollen enters the local landrace. Over generations the landrace carries more and more foreign DNA and loses its distinct type. That is one documented route by which source-region landraces are being diluted.
Don't confuse with
- Backcross: the breeding step; introgression is the overall movement of genes.
- Hybrid: the first cross; introgression requires further generations.
- Contamination (seed purity): stray pollen in a seed run; it can start introgression if the seed is kept.
Germplasm is living genetic material (seeds, pollen, tissue or plants) kept for breeding, research or conservation, representing the genetic diversity of a crop.A backcross is a cross of a hybrid back to one of its parents (or that parent's line), repeated to recover the parent's traits while keeping one introduced trait.A day-neutral cannabis plant that starts flowering by age, usually a few weeks after germination, regardless of light schedule; the trait is mainly recessive.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.An elite cut is a famous, proven clone-only plant, often held by a few growers and traded rather than sold openly.Hemp is cannabis grown for fibre, seed or other uses and, in many laws, defined as Cannabis sativa with no more than 0.3% THC by dry weight.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.A seed run is a dedicated grow in which selected parents are crossed to produce a batch of seed, as opposed to growing for flower.Linkage is the tendency of genes that sit close together on the same chromosome to be inherited together, rather than independently.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.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.
