Role of Chromosome Blocks in Heterosis and Estimates of Dominance and Overdominance1
Chromosome blocks are the genomic units of genetic transmission in sexual reproduction. We work with chromosome blocks, not individual genes in our conventional breeding and genetic research. Thus, chromosome blocks underpin heterosis and estimates of gene action. Chromosome blocks vary in size according to intensity of linkage (frequency of recombination) and the number of sexual generations (the approach to linkage equilibrium); however, all we usually know is the number of sexual generations. Even in the transfer of single gene traits by backcrossing, we usually do not know how much genetic material is linked to the gene of interest. The latter is sometimes referred to as linkage drag. D.F. Jones clearly recognized the role of chromosome blocks in 1917 when he proposed dominance of linked factors as a means of accounting for heterosis. The proposition is elegant because it acknowledges the cumulative effect of linked dominant genes as transmission units. In the years to follow there was much debate about gene action, and heterosis was sometimes interpreted as true overdominance—single loci at which the heterozygous phenotype exceeds that of either homozygote. Maize (Zea mays L.) researchers were careful to point out that estimates of dominance variance exceeding that for straight dominance could be due to either overdominance or linkage disequilibrium of linked loci with favorable alleles in repulsion phase (pseudo-overdominance). Maize researchers went on to compare degrees of dominance in F2 populations in linkage disequilibrium with populations in F8 through F16 in linkage equilibrium. Estimates for degree of dominance were reduced with the approach to linkage equilibrium indicating that the initial heterosis was more likely due to Jones' dominance of linked factors in linkage disequilibrium, than due to true overdominance. In autotetraploid alfalfa, we reached the same conclusion from results indicating dominant linked factors in chromosome blocks, and not multiple allelic interactions, explained improvement and maximum heterosis. Currently, molecular-marker- facilitated investigations of quantitative trait loci in maize report often finding higher yield in the heterozygote than in either homozygote. Based on past research and the fact that chromosome blocks are the units of sexual transmission, it seems likely that the bulk of these heterozygote effects are due to dominance of linked factors as proposed by Jones. Dominant alleles at different loci complement each other by masking recessive alleles at respective loci. The gene action is nonallelic gene interaction or epistasis. Finally, the cumulative action of genes in chromosome blocks not only explains the breeding behavior of cross-pollinated crops, but also explains the fixation of transgressive traits in self pollinated crops, and the ability of auto- and allopolyploids to conceal deleterious recessive traits.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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