Classical concept

CLASSICAL CONCEPT

  • Gregor Johan Mendel (1865) found that individual traits are determined by discrete “factors” and these factors or elements undergo segregation and independent assortment. These factors are then passed on unchanged (except in arrangement) to offspring thus yielding a very large, but finite number of possible variations.
  • Three botanists, Hugo de Vries, Carl Correns, and Erich Tschermak, observed that the elements responsible for pairs of alternative traits “allelomorphs” in the later terminology of William Bateson (1902), which soon came into general use under the abbreviation of “alleles” segregated randomly in the second filial generation (Mendel's law of segregation), and that these elements were transmitted independently from each other (Mendel's law of independent assortment).
  • Bateson had coined the term genetics for the emerging new field of transmission studies in 1906.
  • Wilhelm Johannsen coined the terms phenotype and genotype, which are now used to indicate the appearance of the individual and its actual genetic makeup, respectively
  • He proposed the notion of gene to replace older terms like factor, trait, and character .
  • Thomas Hunt Morgan and his research group used mutants of the fruit fly Drosophila melanogaster and constructed a map of the fruit fly genotype in which genes, and alleles thereof, figured as genetic markers occupying a particular locus on one of the four homologous chromosome pairs of the fly. He established the chromosome theory of heredity.
  • These basic assumptions allowed that genes were located in a linear order along the different chromosomes, and that the frequency of recombination events between homologous chromosomes, that is, the frequency of crossovers during reduction division, gave a measure of the distance between the genes, at the same time defining them as units of recombination
  • Meanwhile, cytological work had also added credence to the materiality of genes-on-chromosomes.
  • In 1933 Jean Brachet was able to show that DNA is found in chromosomes and that RNA is present in the cytoplasm of all cells.
  • In 1941 Edward Lawrie Tatum and George Wells Beadle show that genes code for proteins.
  • "Gene" is a theoretical term. Like all theoretical terms, its meaning has dramatically changed over and over in time, and it has been defined in so many different operational ways.
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Last modified: Saturday, 17 December 2011, 10:19 AM