Practice question

Dominance hypothesis advanced by Davenport in 1908 and expanded by Bruce, Keeble and Pellew and Jones in 1917 postulates that heterosis results from complementation of deleterious recessive alleles carried in different homozygous lines. Inbred parents accumulate different sets of mildly deleterious recessives at many loci encoding enzymes of basic metabolism, photosynthetic electron transport and stress defense. Hybrid receives functional dominant wild-type alleles from each parent at complementary loci, restoring pathway efficiency and increasing fitness. Data from maize whole-genome sequencing show enrichment of loss-of-function mutations in inbreds complemented in hybrids. Hypothesis predicts heterosis magnitude proportional to number of loci with directional dominance and decline upon inbreeding as recessives become homozygous. However alone does not explain persistence of heterosis after purging. 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.