Practice question

Marker-assisted backcrossing comprises two selection layers that accelerate recovery and ensure gene retention. Foreground selection focuses directly on target locus introgressed from donor, using gene-specific or tightly linked markers less than 2 cM to ensure presence of desired allele in each backcross generation. In early generations BC1F1, BC2F1, individual seedlings genotyped for foreground markers, carriers retained, non-carriers discarded, avoiding waiting for phenotypic expression that may require disease inoculation or recessive homozygosity assessment. This prevents accidental loss of target gene during backcrossing where without marker, gene may segregate 1:1 and be lost by chance. For recessive genes, codominant markers differentiate heterozygous carrier from homozygous recessive while phenotype cannot in heterozygous state. High-throughput PCR allows screening thousands of seedlings at seedling stage, accelerating introgression of genes like Sub1 for submergence, xa13 for bacterial blight, and crtRB1 for provitamin A. Foreground selection ensures high transmission of donor allele through backcross pedigree efficiently. Recombinant selection using flanking markers on both sides of target gene helps minimize donor segment length, breaking linkage drag by identifying double crossover events that retain small donor interval containing only beneficial allele while replacing surrounding chromatin with recurrent parent alleles during backcrossing generations for quality recovery.