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Many biologists and other academics held to the idea of blending inheritance during the 19th century, prior to the discovery of genetics. Blending inheritance was merely a widespread hypothetical model, rather than a formalized scientific theory (it was never formally presented to a scientific body, nor published in any scientific journals, nor ascribed to any specific person), in which it was thought inherited traits were determined randomly, from a range bound by the homologous traits found in the parents. The height of a person, with one short parent and one tall parent, was thought to always be of some interim value between its two parents' heights. The shortcoming to this idea was in how it required the person of interim height, in turn, to then become one of the limiting bounds (either upper or lower) for future offspring, and so on down the entire lineage. Thus, in each family, the potential for variation would tend to narrow, quite dramatically, with each generation, and, so it would go for the entire population with every trait. If blending inheritance were true, in this example, all members of a species would eventually converge upon a single value for height for all members. In short, "blending inheritance is incompatible...with obvious fact. If it were really true that variation disappeared, every generation should be more uniform than the previous one. By now, all individuals should be as indistinguishable as clones."〔(Dawkins, Richard, The Guardian, Saturday 8 February 2003 )〕 In addition, blending inheritance failed to explain how traits that seemingly disappeared for several generations often reasserted themselves down the line, unaltered. Blue eyes and blond hair, for example, often could disappear from a family's lineage for several generations, only to have two brown-haired, brown-eyed parents give birth to a blond, blue-eyed child. If blending inheritance were fact, this could not be possible. ==History== The shortcomings, like those mentioned above, of the blending inheritance model were not completely lost to every 19th century thinker, despite the predominance of this hypothesis at that time. In fact, these inadequacies made for an atmosphere in which many equally unconvincing 19th century "arm-chair" hypotheses to be formulated and circulated in attempts to explain inheritance more adequately (see inheritance of acquired characters, maternal impression, telegony, preformationism, Geoffroyism, Lamarckism). Pangenesis was Charles Darwin's Lamarkian attempt to explain inheritance. But despite Darwin's misguided Lamarkian leanings, he rightly had strong doubts about the blending inheritance hypothesis, as evidenced in his private correspondence: In a letter to T.H. Huxley, dated November 12, 1857, Darwin wrote:
In a letter to Alfred Wallace, dated February 6, 1866, Darwin mentioned conducting hybridization experiments with pea plants that were not unlike those done by Gregor Mendel:
Moreover, Darwin's reservations regarding blending inheritance were further reinforced by the inherent conflict this hypothesis had with Darwin's theory of evolution by natural selection, published in his seminal work On the Origin of Species (1859). This incompatibility was best summarized by Fleeming Jenkin in his critique entitled "Review of 'The origin of species'" published in ''The North British Review'', June 1867.〔(Jenkin, Fleeming, Review of 'The origin of species', The North British Review, June 1867, 46, pp. 277-318. )〕 As a staunch defender of blending inheritance, Fleming attempted to criticize Darwin's proposed process of natural selection, a very slow and gradual process, by noting that any favorable trait that might arise in a lineage would have "blended away" (via blending inheritance) long before natural selection had time to work. Blending inheritance (and the many other competing hypotheses) was, ultimately, dismissed by Gregor Mendel's proposal of theory of ''particulate inheritance'', presented in ''Experiments on Plant Hybridization'' (1865), which remains the basis of the inheritance model in the modern evolutionary synthesis. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Blending inheritance」の詳細全文を読む スポンサード リンク
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