The predominant current-day meaning of genotype is the DNA passed to the organism by its parents. The phenotype is the physical and behavioral characteristics of the organism, for example, size and shape, metabolic activities, and patterns of movement. The distinction between the terms seems straightforward, but their use opens up a range of conceptual and methodological questions. How does an organism’s DNA influence the processes of development of its traits over its lifetime—processes that also involve other influences? How can an organism’s traits be used to identify its DNA sequence? Why are the terms genotype and phenotype used if they simply refer to the DNA and traits? A response to this last question is that the predominant meaning is not the only one. Type connotes a class, which leads to the question of what makes organisms similar enough to be grouped in a genotype as a class? …or in a phenotype as a class? In this sense of the terms, how is the membership of organisms in a genotype manifest in their membership in a phenotype—or phenotypes? How can the membership of organisms in a phenotype be used to identify their membership in a genotype? Type also connotes an abstraction away from the full set of observed characteristics. So what is to be accentuated about a genotype and phenotype as DNA and traits or as classes, especially when asking any of the previous questions? To what extent does the abstraction correspond to the control over biological materials and conditions advanced in experimental biology and agricultural breeding? Over a longer historical scale, given that DNA as the material basis of the genotype was discovered well after the term was coined, to what extent does the concept of an unobservable basis for heredity—the genotype—operating below the surface phenotype perpetuate—or depart from—the tradition, most evident in religious thinking, of positing an agent within any apparent agent so that the living being does not develop without being directed by something else? The starting point this entry takes to these and related questions is to consider the import of the terms and their distinction when they were introduced to English-language readers by Johannsen (1911). The co-existence of three kinds of meaning is evident in this foundational text.
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