Backcross

Backcross is a breeding method used to transfer a specific trait from a donor parent into the genetic background of a recurrent parent. In a backcross, the first cross produces a hybrid (often called F1), which is then mated back to the recurrent parent. The goal is to retain the donor’s trait while recovering most of the recurrent parent’s genetic background, resulting in individuals that differ from the recurrent parent mainly at the locus carrying the desired trait.

In a typical backcrossing program, successive backcross generations are labeled BC1, BC2, BC3, and so on. Each backcross increases the proportion of the recurrent parent genome in the offspring. Without selection, the recurrent genome fraction after n backcross generations is approximately 1 − (1/2)^n; for example, BC3 individuals carry about 87.5% of the recurrent parent genome, while BC4 individuals carry about 93.75%.

Selection methods include phenotypic assessment and molecular approaches. Marker-assisted backcrossing uses genetic markers to track the target trait and to select individuals with minimal donor DNA outside the trait locus, reducing linkage drag and accelerating recovery of the recurrent background. Genomic or background selection can further improve background recovery.

Applications are common in plant breeding, where traits such as disease resistance, drought tolerance, or quality attributes are introgressed into elite cultivars. Backcrossing is also used in animal breeding to introduce specific alleles. Limitations include the time required for several generations and the potential for linked donor DNA near the target locus, which marker-assisted strategies aim to minimize.