A backcross (BX or BC) crosses an offspring back to one of its parents, called the recurrent parent. Each round raises the expected share of the recurrent parent's genome. Breeders use repeated backcrossing to move one trait from a donor into an elite line while keeping the elite line's type.
In practice
Example: an elite female clone is crossed to a male carrying the autoflowering trait. A male from the offspring is crossed back to the same female (BX1), then a selected BX1 male back to her again (BX2), then BX3. Each time the breeder selects offspring that carry the wanted trait and otherwise look like the clone.
"Cubing" is cannabis slang for three backcrosses to a female clone, from the idea of raising her share to the third power. It approaches the clone's genotype but never reproduces her exactly, and linkage drag means some donor DNA around the selected trait comes along.
Expected recurrent-parent share after n backcrosses (starting from an F1 at 50%): 1 - (1/2)^(n+1). BX1: 75%; BX2: 87.5%; BX3: 93.75%; BX4: about 96.9%.
Don't confuse with
- Selfing: pollinating a plant with its own pollen. A backcross uses a different plant (the offspring) as the other parent.
- Outcross: crossing to an unrelated plant.
- Clone: a backcross line is never identical to the recurrent parent; only a cutting is.
An elite cut is a famous, proven clone-only plant, often held by a few growers and traded rather than sold openly.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.The genotype is the set of alleles a plant actually carries, either at one locus or across its whole genome, as opposed to the traits you can see.Linkage is the tendency of genes that sit close together on the same chromosome to be inherited together, rather than independently.F1 is the first generation of offspring from crossing two parents; if the parents are inbred and different, F1 plants are uniform and often show hybrid vigour.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.Pollination is the transfer of pollen onto a female flower's stigmas; in cannabis it is usually by wind, or by hand when breeders pollinate chosen branches to make seed.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.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.Introgression is the movement of genes from one population or type into another through hybridisation followed by repeated backcrossing.P1 is the parental generation: the two original plants crossed to start a breeding project, whose offspring are the F1.Stabilisation is breeding a seed line over several generations, with selection each time, until it reliably reproduces the chosen traits from seed.
