Soft inheritance

Soft inheritance is the term coined by Ernst Mayr to include such ideas as Lamarckism. It contrasts with modern ideas of inheritance, which Mayr called hard inheritance. Since Mendel, modern genetics has held that the hereditary material is impervious to environmental influences (except, of course, mutagenic effects).Richards, E.J. 2006. Inherited epigenetic variation — revisiting soft inheritance. "Nature Reviews Genetics" 7:395-401] In soft inheritance "the genetic basis of characters could be modified either by direct induction by the environment, or by use and disuse, or by an intrinsic failure of constancy, and that this modified genotype was then transmitted to the next generation." [Mayr E. 1980 in "The Evolutionary Synthesis" (eds Mayr E. & Provine WB) Harvard Cambridge, Mass. and London p1–48.] Concepts of soft inheritance are usually associated with the ideas of Lamarck and Geoffroy. The concept of hard inheritance holds sway today.

One of the first statements in favour of hard inheritance was made by the English surgeon William Lawrence in 1819. His ideas on heredity were many years ahead of their time, as this extract shows: "The offspring inherit only [their parents'] connate peculiarities and not any of the acquired qualities". [Lawrence, William FRS. 1819. "Lectures on physiology, zoology and the natural history of man". J. Callow, London. 579pp. There were a number of unauthorized reprints of this work, pirated (in the sense that the author went unrecompensed) but seemingly unexpurgated. These editions also lacked the protection of copyright, and date from 1819 and 1848. Some of them were by quite respectable publishers] This is as clear a rejection of soft inheritance as one can find. However, Lawrence qualified it by including the origin of birth defects owing to influences on the mother (an old folk superstition). So Mayr places Wilhelm His, Sr. in 1874 as the first unqualified rejection of soft inheritance. [His W. 1874. "Unsere Körperform und das physiologische Problem ihrer Enstehung". Vogel, Leipzig.] [Mayr E. 1982. "The growth of biological thought". Harvard. p695.] August Weismann, in 1883, gave a comprehensive denial of Lamarkism (soft inheritance) and with his distinction between germ and soma provided a general ideology of hard inheritance which survives to the present day.

Recent work in plants and mammals on the role of the environment on epigenetic modifications of DNA have led to the argument that inherited epigenetic variation is a kind of soft inheritance.

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