On Phenocopies, Their Developmental Physiology and Genetic Meaning

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Abstract
From the experimental production of phenocopies in Drosophila and chicken embryos by treatment with chemical agents, it is concluded that such agents intervene in gene-determined developmental events by preventing them at one point or another from accomplishing their appointed ends, leading thereby to a mutant-like phenotype. The point of interference varies according to the existing genotype, the developmental stage at intervention, kind and intensity of the external agent, and still other factors. With sufficient or the proper kind of pressure many gene-controlled events can probably be made to appear as being the result of sub-threshold alleles, but the existence of such isoalleles as determiners of phenocopies, postulated by Goldschmidt, is not proven and appears unlikely. The phenotypic homologies between mutants and phenocopies represent different ways in which weaknesses of ontogenetic integration are revealed. Specific modifiers occur commonly which, as constituents of the residual genotype, alleviate in a similar manner mutant and phenocopic development.