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History of Genetics - Gregor Johann Mendel

cell egg hybridization traits

Raised on a farm in rural Heinzendorf in German-speaking Silesia, the young Johann, as he was christened, had an early education in the practical aspects of horticulture and agriculture before he left home to attend a gymnasium and at eighteen years of age entered the University of Olmütz. The stress of inadequate finances there led him to apply in 1843 to the Augustinian monastery in Brünn, the industrial and economic capital of Moravia. It was there that, supported by the wise and understanding abbot Cyril Napp, he was encouraged to institute experiments analyzing the phenomenon of hybridization. The subject was of concern to the monastery, with its extensive agricultural holdings in Moravia, a region known for its sheep and wine. Also thanks to the abbot, Mendel had spent two years at Vienna University, studying principally physics, botany, and zoology. There he had learned about the cell theory, according to which the organism is composed neither of a continuous fabric like lacework nor of a multitude of globules but of individual vesicles or cells, all formed by division of preexisting cells that can be traced back to the foundation cell or fertilized egg, this having arisen by fertilization of the egg cell by one pollen cell.

The experiments that led to his well-known theory began with the testing of thirty-four varieties of the edible pea (Pisum) followed by eight years of hybridization (1856–1863). Taking seven traits, he followed the hereditary transmission of each. The scale of the research was unprecedented, the size of his progeny populations being such that clear statistical regularities emerged. It was not just that he noted the separate behavior of the seven traits he studied, nor that there was a marked difference between the population sizes of those carrying the two contrasted characters, but that they approximated to the ratio 3:1. Thus for the trait seed color, Mendel harvested 6,022 green seeds and 2,001 yellow from his hybrid progeny, offering the most striking example among his seven traits of a 3:1 ratio. Further research revealed that two-thirds of the larger class did not breed true and the other third did. Thus the ratio 3:1 was really constituted of three classes in the ratio 1:2:1.

As a physicist trained in combinatorial mathematics this ratio reminded him of the binomial equation (A a) 2 1 A22 Aa 1 a 2. Using A and a to represent the potential carried in the pollen cell and in the egg cell, and knowing that A obscures (is dominant over) a, this expansion appears as 3 A 1 a. Well-grounded in cytology, he suggested that the differing elements brought together in the hybrid remain together until the germ cells are formed. Then they separate and pass into separate germ cells. There result, he declared, "as many sorts of egg and pollen cells as there are combinations possible of formative elements." These claims—known as "germinal segregation" and the "independent assortment" of characters—he supported from his crossing of plants differing in two and in three traits. These two principles were later to be called Mendel's two laws.

Mendel's work did not meet with an enthusiastic response because it was opposed to several securely held beliefs. All the traits he included in the data concerned nonblending characters, but the consensus was that blending is the rule and that the agreed representation of heredity is in terms of fractions: one-half being from the two parents, one-quarter from the four grandparents, and so on, implying that no contribution is entirely lost but that there is a repeated dilution of differences in reproduction. Mendel's theory denied this, for in his theory, after segregation, the elements in question would either be present or absent in a given pollen cell or egg cell, and it would be a question of chance as to which elements finished up in the foundation cell of the offspring.

His paper was directed to two specific questions: First, whether hybridization can lead to the multiplication of species; and second, what part hybridization plays in the production of variation. On the first of these he was clear that Pisum does not yield constant hybrids—that is, hybrids that breed true, reproducing the hybrid form like a pure species. Therefore he instituted experiments with other species to test the general validity of his results. As to the second question, he explained how the variation following hybridization can be understood as the result of the recombination of independently transmitted characters brought together in the hybrid. Therefore he opposed those who, like Darwin, attributed such variation ultimately to the act of bringing together species that have been exposed to different conditions of life. Those who claimed, like Darwin, that cultivated plants are more variable than wild ones due to their changed conditions of life, he also strongly opposed.

History of Genetics - Rediscovery [next]

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over 5 years ago

thanks. that helped my science assignment...but i was confused with the binomial equation though. Was A the dominant allele and the a the recessive one?

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about 3 years ago

History of Genetics - Gregor Johann Mendel