Wednesday, September 19, 2007

Gregor Mendel (1822-1884)

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The value and utility of any experiment are determined by the fitness of the material to the purpose for which it is used, and thus in the case before us it cannot be immaterial what plants are subjected to experiment and in what manner such experiment is conducted.


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Biologist and botanist born in Heinzendorf, Austria. Mendel's first presentation was on his eight years of experimentation with artificial plant hybridization. He entered an Augustinian cloister in 1843 and was ordained a priest in 1847. His experiments in hybridity in plants led to the laws of segregation and independent assortment. These principles have formed the basis for modern genetics.

During his childhood Mendel worked as a gardener, and as a young man attended the Olmutz Philosophical Institute. In 1843 he entered a Augustinian monastery in Brunn, Austria. He was later sent to the University of Vienna to study. By both his professors at University and his colleagues at the monastery, Mendel was inspired to study variance in plants. He commenced his study in his monastery's experimental garden. Between 1856 and 1863 Mendel cultivated and tested some 28,000 pea plants. His experiments brought forth two generalizations which later became known as Mendel's Laws of Inheritance (see below). Ironically, when Mendel's paper was published on 1866 in Proceedings of the Brunn Society for Natural History, it had little impact. It wasn't until the early 20th century that the enormity of his ideas was realized. In 1902, his work was finally rediscovered by Hugo de Vries, Carl Correns and Erich von Tschermak. Mendel died January 6, 1884 in Brunn, Austria.

Mendelian inheritance

Before Gregor Mendel formulated his theories of genetics in 1865 the prevailing theory of inheritance was that of blending inheritance, in which the spermatozoan and egg of parent organisms contained a sampling of the parent's "essence" and that they somehow blended together to form the pattern for the offspring. This theory accounted for the fact that offspring tended to resemble both parents, but failed to show how diversity could be maintained over many generations without all members of a population eventually averaging themselves out. Mendel proposed instead a theory of particulate inheritance, in which characteristics were determined by discrete units of inheritance that were passed intact from one generation to the next. These units would later come to be known as genes, though Mendel did not coin the term himself. Mendel based his theory on studies of inheritance patterns in garden peas (Pisum sativum), which were useful because they could be both cross-pollenated between two plants or self-pollenated with just one. Based on many years of careful, tedious breeding experiments, Mendel developed several fundamental laws of Mendelian inheritance.

Mendel's Law of Independent Assortment

The most important principle of Mendel's Law of Independent Assortment is that the emergence of one trait will not affect the emergence of another. While his experiments mixing one trait always resulted in a 3:1 ratio between dominant and recessive phenotypes, his experiments with two traits showed 9:3:3:1 ratios. Mendel concluded that each organism carries two sets of information about its phenotype. If the two sets differ on the same phenotype, one of them dominates the other. That way, information can be passed on through the generations, even if the phenotype is not expressed.

Mendel's findings allowed other scientists to simplify the emergence of traits to mathematical probability. A large portion of Mendel's spectacular findings can be traced to his proper usage of the scientific method. His choice of peas as a subject for his experiments was extraordinarily lucky. Peas have a relatively simple genetic structure. Also, Mendel could always be in control of the plants' breeding. When Mendel wanted to cross-pollinate a pea plant he needed only to remove the immature stamen of the plant. In this way he was always exactly sure of his plants' parents. Mendel made certain to start his experiments only with true breeding plants. He also only measured absolute characteristics such as color, shape, and position of the offspring. His data was expressed numerically and subjected to statistical analysis. This method of data reporting and the large sampling size he used gave credibility to his data. He also had the foresight to look through several successive generations of his pea plants and record their variations. Without his careful attention to procedure and detail, Mendel's work could not have had the impact it made on the world of genetics.

Mendel's Law of Segregation

Mendel's Law of Segregation essentially has four parts.
  • Alternative versions of genes account for variations in inherited characters. This is the concept of alleles. Alleles are different versions of genes that impart the same characteristic. Each human has a gene that controls height, but there are variations among these genes in accordance with the specific height the gene "codes" for.
  • For each character, an organism inherits two genes, one from each parent. This means that when somatic cells are produced from two gametes, one allele comes from the mother, one from the father. These alleles may be the same, or different.
  • If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. Today, we know several examples that disprove this "law", e.g. Mirabilis jalapa, the "Japanese wonder flower" . This is called incomplete dominance. There is also codominance on a molecular level, e.g. people with sickle cell anemia, when normal and sickle-shaped red blood cells mix and prevent malaria.
  • The two genes for each character segregate during gamete production. This is the last part of Mendel's generalization. The two alleles of the organism are separated into different gametes, ensuring variation.
During his experiments, Mendel encountered some traits that did not follow the laws he had encountered. These traits did not appear independently, but always together with at least one other trait. Mendel could not explain what happened and chose not to mention it in his work. Today, we know that these traits are close together on the same chromosome.

Mendel's First Law

Each adult pea plant has two genes - a gene pair - for each characteristic. The two memebers of each gene pair separate (segregate) randomly into the eggs or sperm of the plant, so that each egg or sperm contains only one member of each gene pair. The offspring therefore inherits one randomly selected gene from each parent for each characteristic.

Mendel's Second Law

During the formation of sperm and egg, the segregation of alleles for one gene is independant of the segregation of alleles for another gene. This law was slightly more complex to demonstrate, requiring the statistical analysis of offspring of plants that differed in two separate characteristics. [This article is licensed under the GNU Free Documentation License and uses material adapted in whole or in part from the Wikipedia article on Gregor Mendel.]

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