Because toxin concentration in pesticide residues and even in transgenic insecticidal crops can vary over time, dominance and selection on the targeted pest are dynamic conditions. Figure 5.5 shows how the toxin dose can alter the survival of heterozygotes (the phenotypic response), and therefore, the functional dominance of resistance to the toxin. For example, several bioassays indicate that the dominance of resistance to toxins decreases as toxin concentration increases ( Tabashnik et al., 2004). The dominance of resistance depends on the environment experienced by the arthropods, including the dose of toxin ( Roush and Daly, 1990). For cases with intermediate heterozygote fitness, the simplest way to model relative fitness for heterozygotes is with a function similar toĪlthough dominance is often heuristically described as a constant genetic property, this is not true for real situations ( Bourguet et al., 2000). However, in the heterozygous state, the interactions between the wild-type protein, still produced by the single S allele, and the toxin are enough to cause mortality. For example, a change in a target site means that the toxin can no longer bind to the given receptor. In contrast, recessive resistance is often associated with a loss of function. One allele causing a gain in function may, therefore, provide resistance. For example, detoxification enzymes are produced at a higher level, thereby allowing more of the toxin to be detoxified.
Typically, nonrecessive resistance occurs through a gain of function the organism can now do something it could not before. Additive expression of resistance causes a genotype to produce a phenotype that is intermediate to the phenotype of either of its homozygous parents. For example, when susceptibility to a toxin is dominant, Sr, the phenotype is vulnerable to the toxin, but when the susceptibility is recessive, sR, the phenotype is resistant.
For a single gene, one allele is dominant over a second allele when the expression of the first allele determines the response of the heterozygote to its environment. Genetic dominance describes the degree to which the phenotype for a heterozygous genotype resembles the phenotype of a homozygous genotype with which it shares an allele.