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Figure 9.12 (continued)

trait that is the product of 100 loci. Even though 90 of the loci start out fixed for one allele, mutation at these loci over time produces enough additive genetic variation to maintain trait variation and a linear response to selection over many generations. Ultimately, the steady-state level of additive genetic variation for a quantitative trait is a product of genetic drift, mutation, and selection all acting on the loci that cause variation in the trait. Because of this, long-term response to selection will depend on numerous parameters in a population such as the effective population size, the mutation rate, and the selection differential along with the number and distribution of effects of the loci that underlie the quantitative trait.

The Illinois Long-Term Selection experiment provides one of the longest records of continuous response to selection for the phenotypes of oil content and percentage of protein in corn kernels (reviewed by Moose et al. 2004). This long-term selection experiment was initiated in 1896 with the goal of determining whether artificial selection could be used to develop strains of corn with kernel phenotypes that were improved from the perspective of animal feed and crop processing. Divergent selection for both higher and lower oil and protein content has been practiced for over 100 generations by using the highest and lowest scoring 20% of ears each generation to form the next generation (Fig. 9.13). From an initial value of 4.7% oil content, response to

Figure 9.13

Phenotypic means for oil and protein content for high and low selected lines of the Illinois Long-Term Selection experiment initiated in 1896. Response to selection has been nearly linear over time, consistent with additive genetic variation that is caused by a relatively large number of loci each with a small effect on the phenotype. The low-oil-selected line was discontinued in generation 89 because oil content was too low to be measured reliably and plants had poor viability. Data kindly provided by J.W. Dudley and S.P. Moose.

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