Practice question

Codominant markers reveal both alleles at locus simultaneously, displaying two distinct bands in heterozygote versus one band in each homozygote on gel or capillary electropherogram. This dosage information is invaluable in marker-assisted breeding because breeder can identify heterozygous carriers Aa that contain one copy of recessive or dominant donor allele while retaining recurrent background, and distinguish from homozygous donor AA that may exhibit drag penalty or lack recurrent adaptation. In backcrossing, BC progeny segregate 1:1 Aa:aa, and only heterozygous carrier advances, allowing precise tracking of introgression without progeny testing that would require additional generation. Dominant markers like RAPD or AFLP show presence-absence, so heterozygote appears identical to homozygote dominant, causing ambiguity and requiring progeny test to infer genotype. Codominance also enables detection of heterozygosity for multiple pyramided genes in same plant, facilitating gene stacking for durable resistance where each gene must be present in heterozygous state in intermediate generations before fixation. Breeding schemes increasingly use multiplexed SNP arrays providing thousands of codominant markers genome-wide, enabling simultaneous foreground, recombinant, and background selection in same assay, increasing throughput and accuracy for pyramiding multiple genes for disease resistance and abiotic stress tolerance in elite backgrounds within short time frame.