External influences always effect the way in which an organism develops. The genetic program, the genotype, stakes out the limits within which changes can be effected by environmental influences. Because the external circumstances that happen to influence each organism’s development are never ever quite identical for two individuals, the form in which an organism actually appears is never exactly the same as that of others with the same genotype. This individual form is called the phenotype (from phainomai, “I appear” in Greek). The changes wrought by external influences, that is to say, the deviations of the phenotype from the genotype, are called modifications. The realization of any genetic program that has evolved during phylogeny is dependent on innumerable external conditions influencing the organism during its individual development, during ontogeny. A modification is any more or less lasting change brought about in an organism by the circumstances of its environment during the course of its individual life. Modifications are omnipresent. There is hardly any small change in environmental conditions that does not cause a slight modification which may last only as long as a few minutes or as long as a lifetime. Two individuals growing up in slightly different environments will always show small differences in their phenotypes.
KeywordsGenetic Program Neural Tube External Influence Open Program External Circumstance
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