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Unformatted text preview: of phenotypes as determined by the shape of the norm of reaction curve. It is as if an observer, standing at the vertical phenotype axis, were seeing the environmental distribution, not directly, but reflected in the curved mirror of the norm of reaction. The shape of its curva- ture will determine how the environmental distribu- tion is distorted on the phenotype axis. Thus, the norm of reaction in Figure 20-8 falls very rapidly at lower temperatures (the phenotype changes dramatically with small changes in temperature) but flattens out at higher temperatures, showing that plant height is much less sensitive to temperature differences at the higher temperatures. The result is that the symmetrical environmental distribution is converted into an asym- metrical phenotypic distribution with a long tail at the larger plant heights, corresponding to the lower temperatures. 20.5 Determining norms of reaction Remarkably little is known about the norms of reaction for any quantitative traits in any speciespartly be- cause determining a norm of reaction requires testing many individual members of identical (or near identi- cal) genotype. In many plants, it is possible to produce identical clones by the simple method of cutting a sin- gle plant into many pieces and growing each piece into...
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.
- Spring '08