15.2: Weldon's critique

Weldon argued that Mendel's focus on traits that displayed dichotomous dominant-recessive behavior in-bred pea strains differed significantly from the behavior of similar plants "in the wild", where traits often vary continuously with a population and unexpected outcomes were observed. He contended that Mendel’s "rules" did not accurately reflect how inheritance works for most traits.^[507]

Most traits are controlled by multiple genes and their alleles often do not act in a simple dominant or recessive manner. Many modern laboratory studies (including Mendel’s) are carried out in in-bred genetic backgrounds, and different phenotypes can emerge in different backgrounds. In in-bred strains the organisms share a common combination of alleles at other genetic loci. Such genotypic homogeneity is an artifact of the way such strains were generated and experiments were conducted. Wild type populations display much more genotypic variation, generally many different alleles of many genes are present. Consider a dominant allele. In the wild, the phenotypic traits associated with that allele often vary - the extent of such variation is characterized through the terms expressivity and penetrance. Variable expressivity refers to the observation that even in the presence of the associated (dominant or homozygous recessive) allele, the phenotypes of individuals vary. As an example, consider a hypothetical pea; the exact degree to which each pea is wrinkled varies – some a little more, others a little less wrinkly – an example of variable expressivity. Similarly, it is possible that out of 100 individuals that carry a particular dominant or homozygous recessive allele, some will not display the trait associated with the allele. The percentage of individuals that display the trait is known as penetrance. Genetic background, together with molecular and cellular stochastic effects influence both the expressivity and penetrance of an allele. Various combinations of alleles of other genes can act to "suppress" and "enhance" the phenotype associated with an allele.^[508] By restricting his work to fully expressive and penetrant dominant and recessive alleles, together with the availability of sufficient numbers of offspring of each class, Mendel was able to make sense of his observations. As noted previously, however, Weldon made explicit the impact of the complexity of genetic interactions, together with environmental and developmental processes that collaborate to shape an organisms' phenotype.

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