Inbreeding is mating between relatives: siblings, parent and offspring, or a plant with itself. It makes offspring more homozygous because relatives share alleles inherited from common ancestors. A population produced this way is an inbred line. The degree of inbreeding is measured by the inbreeding coefficient F, the probability that both alleles at a locus are identical by descent.
In practice
Breeders inbreed on purpose to fix traits and to make parents that pass predictable alleles. The same process exposes recessive defects, so every generation needs culling. Some species tolerate inbreeding well; cannabis, an outcrossing species, often shows inbreeding depression after several generations if selection is weak.
The cannabis term IBL (inbred line) usually implies a finished, stable product. "Inbred line" in general plant breeding just means any population made by inbreeding.
Starting from non-inbred parents, F after one generation of: selfing = 0.5; full-sib mating = 0.25; half-sib mating = 0.125. Repeated selfing: F = 1 - (1/2)^n.
Don't confuse with
- IBL: the cannabis-market label for a stabilised inbred line.
- Line breeding: a milder form of inbreeding around a superior ancestor.
- Clone: copying a plant does not inbreed it; inbreeding needs sexual reproduction.
A plant is homozygous at a locus when it carries two identical alleles there (AA or aa), so it passes the same allele to every offspring.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 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.Inbreeding depression is the loss of vigour, fertility, yield or survival that follows mating between relatives, mainly because harmful recessive alleles become homozygous.An IBL (inbred line) is a seed line that has been inbred and selected over many generations until it breeds true for its main traits, giving uniform offspring.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.Line breeding is a mild form of inbreeding in which plants are mated within a family, often back towards one outstanding ancestor, to concentrate its traits while limiting inbreeding.A sib cross is a cross between siblings from the same seed batch, used to advance a line (F1 × F1 gives F2) while keeping it within the family.
