Sunday, April 8, 2012
Argument-Rats and their environment
Rod Cooper and John Zubek conducted a study on rats' intelligence(27). They started with "Maze-bright" rats that had tested well in mazes for multiple generations and "Maze-dull" rats that had consistently tested poorly for generations with 40% more mistakes. They were raised in three different environments: enriched, normal, and restricted. The expected results were that each rat would get slightly smarter when raised in the enriched environment and slightly dumber when in the restricted environment. However, the results showed that in the extreme environments, the bright and dull rats performed virtually the same. How are these results explained? How does this support the GxE model? Also, explain the role of epigenetics in this scenario.
Alex Casino (Alexjcasino@gmail.com)
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Zubek and Cooper’s classic experiment on the relationship between genetics and environment challenged many preexisting notions on the subject. The experiment revealed that some genetic differences such as rat intelligence were not purely genetic but rather involved a complex interaction between genes and the environment, otherwise known as GxE, in which each component played a critical role in the overall outcome. Such a finding shocked the foundation on which the ideas of the innate genius, such as in the case of Mozart, were built. After Zubek and Cooper’s discovery, many other findings corroborated the implications of their research. These findings include: the relationship between crocodile gender and temperature surrounding the eggs, the relationship between grasshopper color and environment color, the relationship between locus muscle mass and population density (30). These discoveries along with Zubek and Cooper’s discovery all demanded a new explanation in place of the traditional G+E model on the function of genes.
ReplyDeleteSuch an explanation was offered by Harvard Biologist Richard Lewontin. In the traditional model, Genes were represented as unchangeable determinants of the formation of protein, which were also unchangeable entities, which then formed cells, living entities that could respond to the environment. Lewontin proposed an alternative model where Environmental influence came at a much earlier stage, having a direct influence on genes and the proteins they make. Lewontin also insisted that this interaction occurred both ways for genes and proteins also had an impact on the environment (31). What this means is that “if one changes the genes or the environment, the resulting behavior [of an organism] can be dramatically different” (31). This theory can be better understood by understanding the role genes play in creating the traits of an organism. DNA and the genes it makes up can be considered as the blueprints to protein construction. When a cell demands more of a particular protein, it chemically signals RNA polymerase to transcribe the necessary codon portion of a gene to be made into mRNA. This mRNA will then be used as a recipe for ribosomes to construct polypeptides with the help of tRNA and free floating amino acids (Campbell 337). Using this mechanism, the body is able to construct or not construct any protein in response to cellular demands. Thus the body is a product of the demands of its environment. In relation with Zubek and Cooper’s experiment, the difference between maze-bright and maze-dull rats, when disregarding environment, is presumably genetic. Therefore, when considering the knowledge about Gene Expression, the results of the experiment can be explained. Although the maze-bright rats are more genetically talented for solving mazes, this advantage lies only in their genes, which are simply blueprints. Without environmental stimuli, these genes will not be transcribed by RNA polymerase and therefore not turned into protein that accounts for maze-solving prowess. Therefore, when both maze-dull and maze-bright rats were exposed to a non-stimulating environment, their results were nearly identical.
This result supports the GxE model, which suggests a dependent relationship between genes and the environment. In other words, the end product requires the existence of both the right genes and the right environment. If an organism lacks the correct genes, it lacks the DNA to be transcribed for protein production. And if an organism lacks the correct environment, it will not be stimulated to transcribe the genes to produce the right proteins. This necessary coexistence is demonstrated in Zubek and Cooper’s experiment by showing how the desired high level performance in maze rats could not be attained unless both the environmental stimulus and the genes were present. In all other experimental groups, lacking either one or both of the GxE ingredients, the product fell short of the desired result.
ReplyDeleteEpigenetics is the study of inherited changes in phenotype or gene expression caused by mechanisms other than changes in the underlying DNA sequence (en.wikipedia.org/wiki/Epigenetics). In the case of Zubek and Cooper’s study, the effect of the different environmental stimuli on rats with the same DNA would be of interest to the field of epigenetics. The basis of epigenetics lies in the belief that non-genetic cellular memory records developmental and environmental cues which then translate to varied gene expression (http://www.sciencemag.org/content/330/6004/611.short). Specifically, in the rat experiment, the restrictive environment led to the storage of cellular memory in maze-bright rats which led to the non-expression of proteins which facilitated their maze solving. That is one of the multiple examples shown by the experiment that support epigenetics theory.