Mendel, Darwin, Weissmann. In 1865 Gregor Mendel (1822-1884), conducted now classic experiments with the common garden pea Pisum sativum, and defined several basic genetic principles, such as trait segregation and the notion of recessive inheritance, in his explanation of discontinuous variation. Mendel demonstrated that what he called "factors" (elementen, today's genes) are discrete, particulate units that segregate in predictable patterns, even though they may skip generations. Two papers (1866, 1870) and a few letters are all that remain of his scientific experiments, a body of work that was misunderstood and thus infrequently cited during his lifetime. When Mendel's work on discrete variation was rediscovered in 1900, it became the foundation of modern genetics (Dunn, 1991).
The biological explanation of discontinuous variation was, along with transmutation (evolution), one of the intractable biological problems of its day: neither Herbert Spencer's "living units" (1864) nor the hydraulic "gemmules" in Charles Darwin's pangenesis (1868) contained adequate explanations of inheritance. Indeed, Mendel's formulation of particulate "factors" and recessive inheritance is such a profoundly non-intuitive insight that even today's genetic counselors have difficulty explaining these concepts to patients. Embedded within the metaphor of sanguine transmission is an inherent perception that traits that "run in the family" (in the blood) cannot skip generations. Genetic transmission is especially difficult to understand when conditions such as breast cancer can be passed down through unaffected males, or when the kinship system of the family in question conflicts with the pattern of genetic transmission offered by the counselor (Richards, 1997).
It was in The Germ-Plasm: A Theory of Heredity (1893) by the German cytologist and physician August Weismann (1834-1914), that the first accurate (i.e., non-Lamarckian) description of the separation of germ cells from body cells was achieved. Weismann proposed that the germ-plasm is continuous from one generation to the next and is isolated from the soma ("Weismann's barrier"). Weismann's barrier between the germ-plasm and the soma was one of a set of three great eponymous ideas that coalesced at the beginning of the 20th century: the others were Mendelism (segregation and recessive inheritance) and Darwinism (natural selection). This triad of fundamental concepts is prerequisite to any accurate formulation that a disease could be "genetic."
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