#inbreeding depression

Plant Breeding 2

Maize shows which level of inbreeding depression?

Maize shows intermediate sensitivity to inbreeding because despite being predominantly wind-pollinated outbreeder, its genome contains moderate genetic load rather than extreme lethal equivalents seen in alfalfa. Experimental selfing by Shull and East early twentieth century revealed grain yield declining 30 to 50 percent over first three selfing generations, plant height reduced, ear size smaller and days to silking delayed due to exposure of recessive QTL controlling auxin and gibberellin pathways. However inbreds remain viable, fertile and maintainable allowing creation of inbred lines after six to eight generations of selfing with selection against weak types. Moderate depression enabled successful exploitation of heterosis through single cross hybrids producing 50 percent yield advantage over inbreds, foundational to modern maize improvement programs globally. 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.

Ref: Hallauer Quantitative Genetics in Maize Breeding; Duvick Heterosis and inbreeding depression Maize; Singh BD crop classification moderate.

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Plant Breeding 2

Inbreeding depression is highest in:

Magnitude of inbreeding depression correlates inversely with natural selfing rate. Cross-pollinated crops like alfalfa Medicago sativa, carrot Daucus carota and squash Cucurbita pepo naturally outbreed, maintaining high heterozygosity and sheltering many recessive deleterious mutations protected from selection. Forced selfing uncovers this hidden load causing severe biomass reduction, flower abortion and sometimes lethality. Self-pollinated crops like wheat, rice and chickpea evolved under chronic selfing, exposing recessive alleles each generation and allowing purging selection to eliminate severe variants, leaving minimal residual load. Apomictic and vegetatively propagated species circumvent inbreeding via clonal maintenance, thus depression absent. Knowledge of depression level guides breeding method choice: recurrent selection for outbreeders versus pure line selection for selfers, influencing mating design and conservation strategies. 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.

Ref: Allard Principles Plant Breeding mating system depression; Nature Reviews Inbreeding in crops; Singh BD classification cross vs self.

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Plant Breeding 2

Inbreeding depression is caused mainly due to:

Genetic load comprising numerous mildly deleterious partially recessive alleles dispersed across coding regions for enzymes in photosynthesis, respiration and hormone biosynthesis remains hidden in heterozygous condition due to dominance masking. During random mating most individuals heterozygous Aa, functional allele compensates. Inbreeding increases homozygosity, exposing aa genotypes expressing reduced fitness such as chlorosis, dwarfing or sterility. Estimates in Drosophila and maize suggest each diploid carries hundreds of lethal equivalents affecting quantitative traits additively. Purging through selection removes severe alleles but small effect alleles persist due to drift and linkage. Molecular cloning of inbreeding depression QTL in rice shows clusters of nonsynonymous substitutions in defense genes. Hence primary cause remains expression of harmful recessives rather than dominance of beneficial 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.

Ref: Charlesworth D Nature Review Genetics recessive load; Falconer Quantitative Genetics Ch Inbreeding; PubMed genomic basis inbreeding depression.

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Plant Breeding 1

Reduced vigor due to repeated selfing is known as:

Inbreeding depression emerges as phenotypic deterioration in vigor, height, fecundity and stress tolerance following repeated selfing of naturally outcrossing species. Underlying genetic architecture includes dominance hypothesis: many loci carry partially recessive deleterious mutations in genes governing chlorophyll biosynthesis, auxin transport, root elongation and defense signaling. Increased homozygosity makes these recessives expressed, reducing enzyme activity and hormone levels. Pseudo-overdominance where two linked loci in repulsion mimic overdominance also contributes. Quantitative studies in maize reveal hundreds of loci with small negative dominance effects cumulating to yield loss. Because self-pollinated crops have purged much load historically through natural selfing, their depression minimal compared to cross-pollinated species where load sheltered in heterozygotes, making management critical for hybrid breeding. 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.

Ref: Charlesworth & Willis Annu Rev Ecol Inbreeding depression; Falconer Quantitative Genetics; Singh BD Inbreeding depression cause.

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