Principles of Genetics and Breeding

1.1.3.1 Classical genetics

• Gregor Mendel, an Austrian monk conducted a long series of experiments on pea plants (Pisum sativa) during the 1850s and 1860s. In his studies, Mendel recognized that characteristics were inherited as discrete units, and that each of these was inherited independently of the others. He speculated that each parent has pairs of these units but passes only one to an offspring. He also noted that certain forms of one trait were always dominant over others. Today the units that Mendel described are known as genes. Mendel's work went largely unnoticed for more than three decades. In the year 1900, however, Dutch botanist Hugo Marie de Vries, German botanist Karl Correns, and Austrian botanist Erich Tschermak independently rediscovered the Mendel’s works and verified his conclusions.
• Advances in cytology, the science of the structure and function of cells, enabled scientists to more deeply appreciate Mendel’s work. In 1902, American biologist Walter S. Sutton and German cell biologist Theodor Boveri separately noted the parallels between Mendel’s units and chromosomes. The demonstration of the chromosomal basis of inheritance gave rise to the modern science of genetics.
• The term genetics was coined in 1905 by British biologist William Bateson.
• The terms gene and genotype were coined in 1909 by German scientist Wilhelm Johannsen.
• In 1905 American biologists Edmund B. Wilson and Nettie Stevens independently discovered and identified the sex chromosomes. Wilson discovered the X chromosome in a butterfly, and Stevens discovered the Y chromosome in a beetle. The discoveries of the X and Y chromosomes helped scientists begin to unravel new patterns of inheritance.
• In 1910, American biologist Thomas Hunt Morgan identified the first proof of a sex-linked trait, an eye-color characteristic that resides on the X chromosome of fruit flies. With this finding, Morgan became the first scientist to pin down the location of a gene to a specific chromosome.
• Morgan was also the first to explain the implications of linkage, unusual patterns of inheritance that occur when multiple genes found on the same chromosome are inherited together.
• A student of Morgan’s, American biologist Alfred Sturtevant, found early evidence of the mechanisms of crossing over, the phenomenon in which chromosomes interchange genes.
• More definitive proof emerged in the 1930s with work by American geneticists Harriet Creighton and Barbara McClintock. The pair demonstrated gene recombination with experiments on seed color in corn. McClintock later gained notice for her work on transposable elements, large genetic segments that move within a chromosome or even between chromosomes. Her research into these elements, commonly known as jumping genes, earned McClintock the 1983 Nobel Prize in physiology or medicine.