Mendel: Experiments in Plant Hybridization - Essay Example

Mendel: Experiments in Plant Hybridization Introduction While upon initial inspection Gregor Mendel’s Experiments in Plant Hybridization appears unrelentingly complex, upon further analysis it’s discovered that the scientific paper he constructed in response to these experiments follows a logical, as well as rhetorical pattern. When explicating Mendel’s scientific paper it’s important to note that the paper consists of a course of many experiments with slight variations, and to explore them all in procession. As a result this essay explicates the essential elements of Mendel’s paper through this successive means, exploring the concepts of segregation, and the law of independent assortment in the process. Furthermore, the essay also considers the criticisms levied against Mendel’s experiments by noted geneticist R.A. Fisher, and also examines criticism relating to the paper’s rhetorical structure. Survey The opening sections of Mendel’s paper examines past scientific formulations, with Mendel noting that while detailed experiments had been undertaken, nothing in-depth to the extent that it would allow the development of a predictive genetic structure over multiple generations. The first section in which Mendel details the tenants of his experiment is titled ‘Selection of the Experimental Plants.’ In this section Mendel indicates the specific elements he used in selecting plants for his experiment and why these plants are necessary to achieve sound results. Mendel indicates that the characteristics in the plants must be objectively determined (long vs. short stems), as well as observed in every generation (for instance color), and finally they must be able to be bred in a controlled environment. Finally, Mendel indicates that he has chosen a pea plant named a Pisum, as it fits these distinctions. In the next section Mendel details the purpose of his experiment. He states that he intends to investigate the variety in traits that are passed down over the series of generations from the breeding of plants. He distinguishes seven specific traits that he will be observing, chosen for the reasons articulated above. Mendel indicates that past experiments have determined that when two plants with differing traits are bred they exhibit traits from only one plant, not a combination or blending of the two. Mendel refers to the trait that appears in the newly bred hybrid plant as the dominant trait, whereas the trait that isn’t physically detectable the recessive trait. The first experiment section is titled ‘First Generation From the Hybrids’. This section examines what occurs after the two initial plants have bred and produced a hybrid plant. Mendel allows the hybrid plants to self-pollinate and records the traits that appear in the newly produced plants. He determines that the plants exhibit both dominant (traits from the parent hybrid) and recessive traits (traits that were suppressed in the parent hybrid but appeared in one of the original plants). He collects detailed data concerning this occurrence and determines that the ratio of dominant to recessive hybrids as 3:1. This can be understood as the law of segregation. From this he determines that there exist a parental dominant that only exhibits dominant traits and a hybrid dominant that exhibits either dominant or recessive (as such the hybrid dominant can only be identified through generational experiments). The next experiment – ‘The Second Generation From the Hybrids’ – examines what occurs during the next generation of pollination. His results indicate that the hybrids that the earlier hybrids that possessed recessive traits continued to possess recessive traits. The remaining plants that were bred from plants with dominant traits exhibited either dominant (2/3rds) or recessive (1/3rd) traits. Mendel then reconsiders the question of the first experiment concerning parental and hybrid dominants and determines that a more accurate ratio would be 2:1:1. He states, “The ratio 3:1, in accordance with which the distribution of the dominant and recessive characters results in the first generation, resolves itself therefore in all experiments into the ratio of 2:1:1” (Mendel, pg. 12). This ratio accounts for the proportion of hybrid dominants (2) to parental dominants (1) to recessive (1). The next experiment – ‘Subsequent Generations From the Hybrids’ –examines further generations’ traits among the hybrids. For the parental dominant and recessive hybrids they retain the same traits; for the hybrid dominant plants, they produce parent dominants, hybrid dominants, and recessive traits in the ratio 1:2:1. Mendel articulates this ratio as a math equation with the formula A + 2Aa + a. As can be deducted, capital A stands for the dominant trait, while lower case a stands for the recessive. In the next experiment – ‘The Offspring of Hybrids in Which Several Differentiating Characters are Associated’ – is markedly different from the previous experiments in that it examines more than one trait over successive generations. In this experiment he crosses plants with round and yellow seeds with those containing wrinkled and green seeds; the result is the production of round and yellow seeds (the dominant trait). These hybrids are then allowed to self-pollinate and are shown to produce four different kinds of hybrids are produced, with the majority being round and yellow and the minority being wrinkled and green. He then pollinates this generation and discovered that the parental dominant hybrids and the recessive hybrids retained their traits. For the other hybrids he determines that, the offspring of the hybrids in which several essentially different characters are combined exhibit the terms of a series of combinations, in which the developmental series for each pair of differentiating characters are united. It is demonstrated at the same time that the relation of each pair of different characters in hybrid union is independent of the other differences in the two original parental stocks (Mendel, pg. 20). In the final experiment Mendel examines the differences in pollen and egg formations between parental dominant and hybrid dominant plants. In this experiment he produces a series of parental and hybrid offspring. Pollen from the hybrids are then crossed with parental dominants. He then further pollinates these plants and examines their offspring. Here de discovers four distinguishable types of pollen, which should correspondingly produce four different types of offspring. He considers this model in relation to the earlier hybrid ratios he noted, as well as the dihybrid experiment, and indicates that the proportions of the trihybrids can be understood in relation to this as well. The underlining assumption being that the inheritance of one dominant or recessive trait does not affect the type of distribution of an entirely separate trait. While Mendel didn’t state it explicitly, this has come to be understood as the law of independent assortment. Criticism Since the publication of Mendel’s paper there have been a vast number of analytical commentary, as well as direct criticisms of his work. Perhaps the most seminal among the criticisms comes from a school of thought that contends that while Mendel’s conclusions were ultimately accurate; the means that he derived these conclusions may have been slightly fabricated, or the subject of confirmation bias. The most notable of these criticisms was levied against Mendel by another seminal thinker, R.A. Fischer who stated, “the data of most, if not all, of the experiments have been falsified so as to agree closely with Mendels expectations” (Fisher, pg. 132). The crux of Fisher’s argument is that throughout the experiment Mendel was driven by this confirmation bias, so that he ignored anything deviant results that would have indicated otherwise. Fisher contends that, even if done subconsciously, Mendel must have convinced himself that experiments he conducted that didn’t conform to his intended belief must have been conducted wrong and as such discarded them. Furthermore, Fisher argues that Mendel most likely stopped the ratio counts of his experiments when they confirmed his desired result, and not by any objective scientific standard. Ultimately, it’s a criticism of Mendel on the grounds that his experiment didn’t conform to the appropriate scientific rigors required for truly sound data. While it is next to impossible to personally determine the accuracy of Fisher’s claim without carrying out Mendel’s lengthy experiments, there have been a number of written accounts that refute Fisher’s argument. One such piece, written by Hartl, argues that Fisher misinterpreted the exact confines of what Mendel was examining, and was perhaps reckless in accusing Mendel of confirmation bias. Similarly, in a piece written by Novitski it is argued that, while going by Fisher’s interpretation of Mendel’s writing one could accurately detect confirmation bias, in reality Fisher’s understanding and recreation of Mendel’s experiment was erred in a number of fundamental assumptions. In addition to criticisms levied against the scientific plausibility in Mendel’s writing, there are also similar considerations as to the paper’s rhetorical nature. One such consideration was written by Jean Sapp in his essay ‘The Nine Lives of Gregor Mendel’. In the section titled ‘The Rhetorical Nature of Scientific Papers’ Sapp considers the claims made by Fisher, but also notes that past scientists questioned the nature of Mendel’s findings. Sapp writes, “Bateson questioned the authenticity of Mendels celebrated experiments” and then quoting Bateson, “Though his work makes it evident that such varieties may exist, it is very unlikely that Mendel could have had seven pairs of varieties such that the members of each pair differed from each other in only one considerable character” (Sapp, pg. 152). While Sapp doesn’t comment directly on the validity of Mendel’s experiment, an argument is made for the necessity of rhetoric in scientific papers to both better convey necessary material and to hold public interest. Conclusion In conclusion, it’s clear that Gregor Mendel’s experiments on the traits of hybrid plants illustrates to significant and great detail the means by which genetic traits are inherited among successive generations. It’s articulation of the concepts of segregation, and independent assortment became seminal elements for 20th century thinkers in genetics and evolutionary theory. However, perhaps the lasting legacy of Mendel’s Experiments in Plant Hybridization is not due to its function as a cornerstone of genetic theory, but is rather because it is a perfectly crafted piece of literature. Indeed, it is this rhetorical efficiency that led R.A. Fisher to famously deride its scientific validity. Ultimately, the underlining tenants of Mendel’s results have played out accurately, and despite what may be flights of rhetoric, the paper firmly demonstrates the need for the confluence of literary and scientific thought. References Fisher, R. A., (1936). Has Mendels work been rediscovered? Ann. Sci. 1: 115–137. Hartl, Daniel L.; Fairbanks, Daniel J. (1 March 2007). "Mud Sticks: On the Alleged Falsification of Mendels Data". Genetics 175 (3): 975–979. Mendel, G., (1866). Experiments in Plant Hybridization. http://www.esp.org/foundations/genetics/classical/gm-65.pdf Novitski, Charles E. (March 2004). "On Fisher’s Criticism of Mendel’s Results With the Garden Pea". Genetics 166 (3): 1133–1136. Sapp, Jan. "The Nine Lives of Gregor Mendel". In H.E. Legrand (ed.) Experimental Inquiries (Kluwer, 1990) pp. 137-166. Read More