Shenk in the evidence portion of his book contrasts his idea of GxE to the widely known Mendelian version of Genetics. He describes the "built-in logical flaw" of the pea plant experiment because all the experiments were performed in a constant, consistent environment, so this eliminates any perceptible impact the environment could have on the pea plants. Obviously under the same consistent environment, only genetics could have an impact on the growth pea plants of the pea plants (184). Is Mendel's experiment and the idea of genetics determining everything less credible when considering this fact? Is this a flaw in his experimental design? Also, contrast Mendel's idea of genetics with the newly learned idea of epigenetic inheritance and the impact it can made on your genes. Also include how inheritance isn't as simple as how Mendel's pea plant experiment depicted it as (for example, bring in co-dominance, sex-linked genes, linked genes etc.)
(Yanfei Gao, feifeiyg@yahoo.com)
Mendel's experiments in genetics led him to the conclusion that genotype or "the specific allelic combination for a certain gene or set of genes" is directly responsible for producing a specific phenotype or "the outward, physical appearance of a particular trait" (http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel1.htm). It is not surprising Mendel reached this conclusion based on the understanding provided by the GxE model that genes and environment are involved in a dynamic interaction where environmental factors are responsible for activating genes that influence the expression of specific traits. Because Mendel inbred his plants to create a pure line or "a population that breeds true for a particular trait," their genes were so similar that when exposed to the same environment for development, they expressed the same traits (http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel1.htm). Mendel's neglect to take into account the impact of environmental influences on gene expression certainly discredits his findings to some degree. The environmental effects Mendel neglected to understand have recently been explained by epigenetics or the study of environmentally influenced alterations to the wrapping of histone proteins and DNA strands in the epigenome mediating gene expression (Shenk 158-159). As an example of the process of epigenetics, Shenk cites a study of toadflax flowers by Enrico Coen in which two flowers with identical genomes were compared to show one had developed five spurs while the other did not yet they possessed identical flower symmetry genes (Shenk 157-158). Had Mendel compared two of his genetically identical pea plants after growing them in different environments he might have also found a similar variation in gene expression.
ReplyDeleteKate White (kw2020@gmail.com)
Continued
ReplyDeleteThe impact of this understanding of epigenetics and the idea that the alterations to our epigenomes we create through our behavior or environment can be passed to future generations is that a greater responsibility is placed on the individual for either achieving or stagnating in their pursuit of greatness in a field as their genes can no longer be to blame; it also places accountability on the individual for actions that can alter the epigenome in such a way that it negatively impacts future generations such as the study cited by Shenk in which geneticist Marcus Pembrey presented data demonstrating that "cigarette smoking in one generation of humans had effects across several generations" (Shenk 160).
In relation to Mendel's understanding of the genes themselves through the means by which they were inherited, his conclusions were that there are present either dominant or recessive alleles in homozygous combinations or a dominant and a recessive allele in a heterozygous combination where the dominant will be expressed in the phenotype and the recessive will not. While this conclusion may be true in gene inheritance in some cases, more complex patterns of inheritance have also been discovered such as codominance where "neither parental trait is dominant nor recessive [...] the two traits both appear in the offspring, often showing up in different parts of the plant or animal" (http://bridget-coila.suite101.com/incomplete-dominance-and-codominance-a173718). Another such pattern is incomplete dominance which is "when the two alleles inherited from the parents are neither dominant nor recessive, but blend together to give a physical trait that is somewhere between the two" (http://bridget-coila.suite101.com/incomplete-dominance-and-codominance-a173718). These instances of mixed phenotypes could not be explained by Mendel's understanding of simple dominance. Also as environmental factors are now recognized to be a large contributor to how genes are expressed with regards to the GxE model, the complexities involved in the inheritance of genes and the numerous variables in their potential for expression absolutely render Mendel's experimental design obsolete for modern genetics conclusions, however, in its time it was pivotal to the beginnings of the understanding of genetics.
Kate White (kw2020@gmail.com)
Kate concluded that “the inheritance of genes and the numerous variables in their potential for expression absolutely render Mendel's experimental design obsolete for modern genetics conclusions”. I find this statement utterly blasphemous in its attack on Mendel’s findings which are the foundation of modern genetics. Although there have been amendments to some of Gregor Mendel’s findings, the truth is: “[Mendel’s] work has stood the test of time, even as the discovery & understanding of chromosomes & genes has developed in the 140 years after he published his findings. New discoveries have found "exceptions" to Mendel's basic laws, but none of Mendel's things have been proven to be flat-out wrong” (http://www.hobart.k12.in.us/jkousen/Biology/mendel.htm). Mendel’s experiment has never been more credible.
ReplyDeleteWhile recent epigenetic findings have concluded that Mendel’s discoveries on genetics do not account for all the factors that the complexities involved in gene expression, Mendel’s work was a remarkable landmark for modern science because it predicted the nature of chromosomes during meiosis 50 some years before the first coining of the word “allele” and nearly 90 years before Avery and McCarty showed that genes were encoded within DNA (http://en.wikipedia.org/wiki/Gene#History). It is true that Mendel completely disregarded environmental factors in his pea pod experiment; however, he did this intentionally. Mendel was a pioneer in his field. Like previously mentioned, there was no knowledge of chromosomes at the time the scientific understanding of the concept of genetics was that the process resembled mixing of two types of paint to create a third color“ (Campbell 262, 264) At such a stage, Mendel had no solid foundation to back his study on. So, like a good scientist, he remained true to the experimental method and sought to keep as many variables constant in order to avoid experimental error. One of these wild cards was the environment and, like Mendel predicted, it does impact gene expression quite a bit. In addition, “Mendel chose to track only those characteristics [in pea plants] that varied between two distinct alternatives” (Campbell 264). This way, significant findings in his data were more apparent. If anything, Mendel’s experimental design was quite immaculate and the accuracy of its results quite amazing. Mendel’s goal in conducting the experiment was to discover how sexual reproduction or meiosis functions and he accomplished this elegantly with his pea plant experiment.
Mendel’s study was a basic investigation on the fundamentals of inheritance and the functioning of meiosis. He established fundamental laws such as the law of segregation that stand as pillars of genetics today. However, Mendel was, after all, just a simple, but scientific, monk. His revolutionary pea plant experiment did not delve anywhere beyond the basics of sexual reproduction, nor did it seek to. In contrast, recent research on epigenetics has tackled that complexity by exploring the effects of the environment, the experimental variable that Mendel rendered constant, on gene expression. While Mendelian genetics made a connection directly between inheritance and expression, epigenetics challenges that notion with the idea of GxE. GxE maintains that there is a constant interaction between genes and the environment that can cause variation in whether a certain gene will be transcribed into mRNA and translated into protein or whether it will remain dormant. This shifts modern genetic theory beyond, not away from, Mendelian interpretations by offering possibilities such as two blue eyed parents producing a brown eyed child.