The Argument: In Chapter 1, Shenk relates the GxE model to
indoctrinations continually emphasized in Mendelian genetics in order to
clarify the relationship between the environment and the genetic composition of
an organism. While Shenk acknowledges the fact that Mendel’s discoveries upon
experimentation with pea plants proved the existence of heritable genetic
material that could predictably influence phenotypes, his experiments only
revealed half of the picture. By maintaining constant environmental factors,
the opportunity for phenotypic variation due to external stimuli was eliminated,
voiding the interactions present within the GxE model that dictates phenotype
in natural environments. Explain how and why the phenotypes of Mendel’s peas
remained relatively predictable via genetics, relating your response to both
Shenk’s GxE model and to Chapter 14 of Campbell (Mendel and the Gene Idea).
Relate your response to the biological theme of interdependence in nature, as
an organism’s phenotype is the outcome of the complex interactions between its
genetic composition and external stimuli. Upon later discussing the origin of
the GxE model, Shenk cites an essay in New
York Times Magazine and its referral to the influence of genes on
phenotypic traits being “probabilistic”. Explain how this terminology is
inaccurate and propose another term to describe genes’ effect on phenotype by
using the GxE model.
Nick Sotos (nsotos13@gmail.com)
Mendel's experiment with pea plants produced data that were uniformly accurate to genes because he kept environmental stimuli constant. Each pea plant expressed the gene that was supposed to be expressed because the environment was not a factor in the experiment. This explains why the results for Mendel's experiment was so accurate to the genes each pea plant had. Shenk points out Mendel's mishap with believing genes control everything in a person: "Though Mendel couldn't detect it with perfectly calibrated pea-plant hybrids, genes are not like robot actors who always say the same lines in the exact same way" (19). Shenk calls forth the notion that Mendel missed a key factor in his “heritable factors” in shaping a person. Mendel didn’t recognize the importance of environment - a very significant part of development. Campbell also makes clear that Mendel only chose to follow traits that had “varied between two distinct alternatives” (263). This allowed Mendel to easily calculate the inheritance of each pea plant. The phenotypes could either by a white or purple plant. It could have a long or short stem. It could have a yellow or green seed. (http://anthro.palomar.edu/mendel/mendel_1.htm). The little variation made Mendel’s experiment possible in identifying phenotypes caused by the genotype. Shenk talks about how Mendel was misguided by his experiment because the environmental factors were held constant, and environment plays a major role in shaping phenotypes (20). The pea plants have a huge dependence on nature because it is nature that acts on organisms that changes phenotypes as well as genes.
ReplyDeleteShenk is inaccurate when using “probabilistic” when describing genes’ effect on phenotypes because it doe snot have to do with probability. Phenotypes are not created from the “probable” gene that its has. The environment plays a crucial role in shaping the phenotype while working alongside genes. Instead, phenotypic effects from genes should be depicted as a component. Genes are just one of many key components that influence phenotypes in an organism. Just like symbiosis where animals interact, genes influenced by the environment create different phenotypes for each organism.
Josh LeVay (blevay@comcast.net)
This comment has been removed by the author.
ReplyDeleteAccording to simple Mendelian inheritance, each trait has two alleles (alternative versions of a gene), a dominant and a recessive. The offspring would receive a single allele from both parents, either dominant or recessive (Campbell 265). A dominant allele is the form of a gene that dictates the phenotype of that particular gene if at least one is inherited (homozygous dominant or heterozygous genotypes), while a recessive allele is the form that has no noticeable effect on the phenotype unless the recessive form of the gene is inherited from both parents (homozygous recessive genotype) (Campbell 266). This was Mendel's reasoning behind the predictability of the various traits he observed in his pea plants.
ReplyDeleteBut with Shenk's new GxE model, we now realize that Mendel's theory of inheritance is flawed and does not include environmental factors at all. According to Shenk, "the more complex the trait, the farther any one gene is from direct instruction" (26) and clearly, simple traits such as flower color, seed color, and seed shape are extremely straightforward. These traits are simple enough that these traits are described as "near-Mendelian" even under Shenk's GxE model. However, even these simple traits have exceptions as John Hopkins genetecist Victor McKusick verifies, "two blue-eyed parents can produce children with brown eyes" (24). Such a phenomenon is impossible under Mendelian inheritance because the allele for blue eyes is recessive. Therefore, both parents would have to have two recessive alleles for blue eyes and only be able to pass on the blue eye allele to their offspring. The interdependence between the environmental stimuli and genes is what affects the phenotype of any trait. In a study, single Asian and African water flea was cloned to form two genetically identical organisms. One of the clones was then exposed to chemical cues from predaceous fish while the other was not. What the scientists found was that the clone exposed to the chemical cues grew a sharp helmet and extended tail spine when compared to the control flea. http://faculty.bennington.edu/~sherman/advanced%20physiol%20/evol%20and%20costs%20of%20phenotypic%20plasticity.pdf
This clearly shows that even physical traits cannot be described as "probabilistic" as all traits are a combination of both genes and environmental stimuli. Shenk describes genes as a set of "knobs and switches [which] can be turned up/down/on/off at any time--by another gene or by any minuscule environmental input" (19). Josh, I believe that it was not Shenk who said genes were probabilistic, but he was quoting the magazine and proving that they were inaccurate.
-Josh Choi (joshchoi95@yahoo.com)