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History Nature of the Gene - From Elements To Genes

pairs characters traits inheritance

In 1865, when Mendel articulated the laws of inheritance that now bear his name, he did not use the terms "gene" or " allele." He referred instead to "elements" and "characters." Mendel described the patterns of inheritance he observed in terms of character pairs. These pairs segregate to form the next generation of character pairs and remain independent of the behavior of other character pairs. The external characters of the pea plants he described corresponded to elements within the germ cells of the same plants. Whether Mendel thought that pairs of characters were expressions of pairs of cellular elements is not clear.

Edward Tatum. He and his colleague George W. Beadle developed the "one gene-one enzyme" model of gene function.

By the time of Mendel's rediscovery in 1900 by Hugo de Vries, Carl Correns, and Erich Tschermak, however, visible characters were understood to be expressions of hereditary particles within each cell. Just as characters occurred in pairs, Mendelians interpreted hereditary particles as occurring in pairs. William Bateson called these pairs of hereditary particles "allelomorphs," a term that would eventually be shortened to "alleles." The idea that something within the gametes (sperm and egg cells) specified the characteristics of the organism was captured by Johannsen's term "gene."

It was not at all clear in the first decade of the twentieth century that the segregation and assortment of alleles could explain patterns of inheritance. While Bateson forcefully advocated Mendelian principles, many of his contemporaries, such as Karl Pearson and Walter F. R. Weldon, explained patterns of heredity in terms of continuous characters, instead of discrete Mendelian character pairs. For Pearson, Weldon, and other biometricians, as they called themselves, traits were expressed as continuous distributions from tall to short, for instance. The mating of a tall parent with a short parent would yield offspring with a range of heights, as the parental traits were blended together.

Because Mendelians, like Bateson, strongly identified characters with alleles, they insisted that both were discrete. The apparent blending of parental traits was a stumbling block until the distinction between genotype and phenotype was consistently applied and single continuous traits such as height were seen to be the expression of many individual Mendelian factors. Thus, multifactor inheritance allowed geneticists to explain a continuously distributed phenotype as an expression of many discreet Mendelian factors or genes.

History Nature of the Gene - The Chromosome Theory [next]

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