Practice question

Overdominance hypothesis originating from Shull 1908 and East 1908 suggests heterozygous state at individual locus confers intrinsic superiority exceeding either homozygote due to allelic interaction. Mechanisms include formation of heterodimeric enzymes expanding substrate range, such as alcohol dehydrogenase heterodimer with broader catalytic efficiency, balanced gene expression dosage or differential interaction with regulatory proteins. Example includes MHC locus heterozygote advantage against pathogens. At quantitative trait level, overdominance may arise from pseudo-overdominance where two linked loci in repulsion each dominantly favorable mimic single locus advantage. Molecular dissection in rice reveals overdominant QTL qHY where heterozygous transcription factor complex binds wider promoter repertoire increasing tiller number. Hypothesis complements dominance by explaining heterosis even after elimination of deleterious alleles. This understanding supports competitive exam preparation for NEET, GATE and CSIR NET concepts linking genotype with phenotype through molecular pathways involving transcription factors, hormones and metabolic enzymes that regulate development, adaptation and reproductive biology in applied breeding programs.