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Germ plasm () is a biological concept developed in the 19th century by the German biologist August Weismann. It states that heritable information is transmitted only by germ cells in the (ovaries and testes), not by somatic cells. The related idea that information cannot pass from somatic cells to the germ line, contrary to Lamarckism, is called the Weismann barrier. To some extent this theory anticipated the development of modern genetics.
However, a careful reading of Weismann's work over the span of his entire career shows that he had more nuanced views. He insisted, like Darwin, that a variable environment was necessary to cause variation in the hereditary material. Because genetic information cannot pass from soma to germ plasm, these external conditions, he believed, caused different effects on the soma and the germ plasm. Thus, the historian of science Rasmus G. Winther states, Weismann was not a Weismannian, as he, like Darwin, did believe in the inheritance of acquired characteristics, which later came to be known as Lamarckian.
The part of Weismann's theory which proved most vulnerable was his notion that the germ plasm (effectively, ) was successively reduced during division of somatic cells. As modern genetics developed, it became clear that this idea is wrong in most cases.For example, by studies of polytene chromosomes in salivary glands (i.e. somatic cells) of larval Drosophila. Cases such as Dolly, the cloned sheep, proved via somatic cell nuclear transfer that adult cells retain a complete set of information – as opposed to Weismann's increasingly determined gradual loss of genetic information – putting this aspect of Weismann's theory to rest as a general rule of metazoan development. However, genetic information is readily lost by somatic cells in some groups of animals through somatic genome processing. The best known example is the , in which the phenomenon of chromatin diminution was first described by Theodor Boveri in 1887.
The idea was to some extent anticipated in an 1865 article by Francis Galton, published in Macmillan's Magazine, which set out a weak version of the concept. In 1889 Weismann wrote to acknowledge that "You have exposed in your paper an idea which is in one essential point nearly allied to the main idea contained in my theory of the continuity of germ-plasm". The Rough Guide to Evolution: Galton or Weismann first to continuity of the germ-plasm?
The Weismannian notion that the germ cells are unaffected by somatic cells or their environment is however proving not to be absolute. Chemical modification of the nucleotide bases that constitute the genetic code such as methylation of as well as modifications of the around which DNA is organized into higher-order structures are influenced by the metabolic and physiologic state of the organism and in some cases can be heritable. Such changes are called epigenetics because they do not alter the nucleotide sequence.
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