Crossing and backcrossing: basic terms in breeding work

Crossing denotes the pollination of a female plant with pollen from a genetically different male plant, producing a new combination (F1 generation).

Backcrossing crosses an already existing hybrid again with one of its original parent lines, to deliberately increase that parent's genome share in the offspring.

An F1 cross is heterozygous at many gene loci (two different allele variants per locus) — this often produces hybrid vigor (heterosis: stronger growth than either parent), but it does not make the line genetically uniform.

When an F1 plant is propagated further with itself or a sister plant (F2), the underlying gene combinations reassort according to Mendelian inheritance rules — visible as markedly wider phenotype spread than in the F1 generation.

An F1 hybrid is backcrossed with one of its parents — the resulting generation is called BX1 (first backcross) and already carries a larger share of the parent genome than the F1.

Repeated backcrossing across several generations (BX2, BX3 …) further increases the parent genome's share step by step, until the offspring come genetically very close to the chosen parent — with the exception of the trait deliberately bred in.

A common example is crossing in the autoflowering trait from a ruderalis introgression into a proven, potent photoperiod elite line: the first cross introduces the trait, and repeated backcrossing onto the elite parent line progressively reduces unwanted ruderalis traits such as reduced growth height.

Backcrossing is also used to bring an already stable, proven line back to its original character after an outcross.

P (parental generation): the original, genetically different parent plants.

F1: first generation from a P cross, mostly genetically uniform in appearance despite internal heterozygosity.

F2: produced by selfing or sibling-crossing the F1, shows visible phenotypic segregation.

Sn (S1, S2 …): generations from repeated selfing of the same line, increasingly homozygous and phenotypically more uniform.

Selling or expecting F1 seeds as a 'stable' line, even though F1 hybrids are genetically heterozygous and segregate in the next generation.

Assuming a fully restored parent line after just one backcross (BX1) — several BX generations are needed for a high parent share.

Confusing backcrossing with simple crossing in terminology, leading to wrong expectations about stability and trait expression.