Mendel's Work: Experiments on Plant Hybridization - Assignment Example

Steven Richardson Biology 276 June, 2006 Mendel’s Work Experiments on Plant Hybridization Before Gregor Mendel came to be known as the Father of Genetics, his major influences were his professors in physics and botany, particularly in the application of mathematics to science and the basis of variation among plants. After graduation, Mendel entered the monastery where he taught and began experimenting on garden pea hybridization and its genetic inheritance. He opted to use garden peas (Pisum sativum and its subspecies) mainly due to its distinctive characteristics and traits. Basically, garden peas are: monoecious plants (where the male and female reproductive organs are separately situated in the plant), self-pollinating, and the pollens can be manually moved from one plant to another through cross-pollination. Mendel had spent numerous years in studying the genetic traits of garden peas through careful observation and comparison of the following seven discrete qualitative traits: (1) smoothness of the seeds; (2) color of the seeds; (3) color of the seed coats; (4) shape of the pods; (5) color of unripe pods; (6) position of flowers; and (7) length of the stems (see Figure 1). One of Mendel’s observations was the invariable difference of one heritable trait to one another, either as dominant or recessive. Nowadays, a dominant trait refers to those traits that are clearly manifested physically (phenotype) in a heterozygous genotype (needs only one copy of the gene from either parent for the trait to manifest, i.e. Aa) while recessive traits are manifested physically in a homogenous genotype (needs two copies of the gene from both parents for the trait to manifest, i.e. aa). In his breeding experiments, Mendel would cross-pollinate (also referred to as hybridization) purebred garden peas of different traits such as a violet flower versus a white flower, full pod versus constricted pod, and the like (Wikipedia, 2006). He made several experiments on varying crosses such as monohybrid, dihybrid, and trihybrid. Monohybrid crosses are being done by mating two garden pea plants with only a single trait (homozygotes for tall stem) or true breeding of peas with opposing traits. Abbreviated as Aa x Aa, monohybrid crosses for round seeds versus wrinkled seeds (called the Parental generation or P generation) had resulted into dominant trait for round seeds. About 100% tall-stemmed peas are also produced when it is crossed with short-stemmed peas. The crosses of the P generation are called F1 generation (or first filial generation) or commonly referred to as offspring. On the other hand, F2 generation offspring are produced when the F1 generation offspring are selfed (selfing) or crossed among one another. The resulting offspring are plants with tall and short stems, usually having a 3:1 ratio (three tall-stemmed plants for every one short-stemmed plant). Other garden pea traits are tested with this type of crossing. With the crossing of monohybrids, Mendel was able to conclude that that: (1) the recessive traits of a gene, aside from the dominant traits, are responsible for the changes in the phenotype; (2) any living organism inherits two alleles from each parent; (3) for the two inherited alleles, one is fully manifested in the phenotype (called dominant) while the other is hidden or completely masked (called recessive); and (4) the two inherited alleles undergo segregation during meiosis (gamete production) which thus explains the Law of Segregation (Thoma, 2006). Dihybrid crosses are carried out for plants that are being observed for two traits. In the case of garden peas, homozygous round yellow seeds (RRYY) are crossed with homozygous wrinkled green seeds (rryy). The dihybrid crossing resulted in the formation of F1 plants that have inherited heterozygous progenies (RrYy) for both traits being observed. Round yellow garden peas are the dominating phenotype of the seeds (Figure 2). To prove that the traits are whether or not independently inherited, the F2 generation should have a segregation phenotypic ratio of 9:3:3:1 (having RY, Ry, rY, and ry respectively with each ratio). These results follow the Mendel’s Law on Independent Assortment. Trihybrid crosses are performed for assessing the degree of assortment for three individual traits within a plant. If monohybrid crosses look like Aa x Aa and dihybrid crosses are abbreviated as AaBb x AaBb, trihybrid crosses are symbolized as AaBbCc x AaBbCc. It follows the same principles of hybridization as that of monohybrid and dihybrid crosses except for the fact that it observes more than two traits. Evaluation of Mendel’s Work The following information discusses some critical analysis of Mendel’s experiments and principles that has brought about the foundation of Modern Genetics. 1. First and foremost, accidental impregnation or contamination by foreign pollen is difficult to quantify during Mendel’s time. This implies that the degree of error in his experiments is not considered statistically in his probabilities and experimentations. Although Mendel has provided proper replication and appropriate significant results, other factors that may have contributed to his results such as the agrometereological factors and other interventions must be carefully indicated. Without proper statistical analysis and quantification degrees of freedom, observation alone is not enough to substantiate the authenticity of such results. 2. The manner of selecting the seeds of garden peas and the criteria for selecting its sources were not properly documented. Mendel only mentioned that, “34 more or less distinct varieties of Peas were obtained from several seedsmen and subjected to a two year’s trial.” This implies that the integrity of the source of the seeds is in question. In modern seed selection, several steps in assessing the viability, germinability, and purity are being undergone prior to the conduct of breeding objectives. Hence, it is not impossible that seed contamination and outcrossing may have occurred during the course of the experiments which opposes Mendel’s claim that false impregnation by a foreign pollen is very slight. 3. Mendel indicated that his experiments were done in pots and garden beds in green houses. With such environments, it would be difficult to assess whether the environment is inducing such phenotypic traits. Otherwise, Mendel’s assumptions, although not explicitly implied, could be proven that the plant regardless of its environment is outwardly expressing the aforementioned traits. In today’s breeding objectives, modern varieties for mass production as well as varieties for seed production are required to be subdued in field experiments and other stress factors prior to its introduction in the market so as to ensure its stability and avoid unnecessary segregation. 4. In reality, it is difficult to obtain Mendel’s phenotypic ratio of 9:3:3:1as characters with Ry and rY are difficult to assess, determine, and evaluate by just relying on optical means. Furthermore, there is a possibility that Mendel might have mistaken some heterozygous traits as homozygous traits. This would lead to obvious biases towards the geared experimental results. 5. Clearly, Mendel’s work is only focused on the assessment and evaluation of qualitative traits, and not on quantitative traits. His reasons may be unknown to many, but the basic consideration of such traits may be deemed highly valuable in assessing the relationship of some qualitative traits with its quantitative counterparts. 6. On the other hand, some critiques argue that the articulation of the two laws of inheritance such as the Law of Segregation and Law of Individual Assortment might have been attributed to Mendel’s work (Fairbanks & Rytting, 2001). As presented in the Mendel’s paper on the Experiments on Plant Hybridization, it only vaguely shows the main thoughts extrapolated on the aforementioned theories. The only hypothesis mentioned is his results and its derived conclusions. Unlike modern breeders, Mendel’s work only provided preliminary baseline date for the development of such laws of inheritance. Whether he coined the terms or not, suffice to say Mendel does not need any direct recognition for such ideas as the mere fact that people knew that the novel ideas came from him is sufficient enough in today’s society. 7. Critics also indicated that Mendel might have detected but failed to mention genetic linkaging. With the type of traits that Mendel focused on was based on previous studies that have been done before him such as those of Kolreuter, Gartner, among others, there is difficulty in assessing whether Mendel had really observed and disregarded genetic linkage. 8. There were also some arguments pertaining to Mendel’s influence on Charles Darwin’s work, or the other way around. Bateson (1913) argued that, “With the views of Darwin which at that time were coming into prominence Mendel did not find himself in full agreement.” Nevertheless, it can be concluded that both highly distinguished individuals found interest in Evolutionary History. In this regard there is evidence to show that despite the language barriers of these two individuals, the issue of evolution has been the subject of man’s thoughts across the miles and physical barriers. One person cannot be summarized as the patron of another, but it is safe to conclude that both influences have taken a large part of modern history. 9. Mendel’s work cannot be cited through its original language and context. As evidently seen from his use of technical terms and language, modern day genetic terminologies are not yet present during his time. Ambiguous or vague terms must not be quoted by the word but through its in-between implications and meaning. 10. Despite various comments on the validity of Mendel’s works, Fairbanks & Rytting, (2001) indicated that, “there is no credible evidence to indicate that Mendel was inaccurate or dishonest in his description of his experiments or his presentation of data.” Such evidences and support being manifested would still be liable towards a never-ending validity and questions from emerging scientists and philosophers. References Bateson, W. Mendel’s Principles of heredity. Cambridge: Cambridge University, 1913. Fairbanks, D. J. and Rytting, B. “Mendelian Controversies: A Botanical and Historical Review.” American Journal of Botany 88(5): 737–752, 2001. Fisher, R. A. “Has Mendel’s work been rediscovered?” Annals of Science 1: 115–137, 1936. Thoma, Sharon. Transmission Genetics: Mendel and The Gene Idea. Madison: Edgewood College, 2006. Wikipedia Encyclopedia. “Gregor Mendel.” Wikipedia: The Free Encyclopedia. Retrieved from website: . Read More