Practice question

Backcrossing is designed to rectify a specific defect in an otherwise elite, adapted cultivar without altering its overall genetic background. A donor parent contributes one or a few desirable alleles, such as a dominant disease resistance gene Yr15, Bt, or bacterial blight gene Xa21, while recurrent parent provides superior yield, quality, and adaptation encoded by hundreds of favorable QTLs. By repeatedly crossing hybrid progeny carrying the target gene back to recurrent parent and selecting for that gene each cycle, breeder introgresses a small donor chromosomal segment. Unlinked donor genome decays by 50% each backcross, restoring elite genome to >99% by BC6. This precision contrasts with pedigree methods that create new variability through broad recombination. It is ideal for adding oligogenic traits to established varieties, near-isogenic line development, and transgene introgression while preserving linkage blocks responsible for elite performance and G×E stability. At molecular level, foreground selection with SSR or SNP markers flanking target gene tracks introgression, while background selection with genome-wide markers minimizes donor chromosome retention and reduces linkage drag around inserted locus significantly.