2. Philosophy - Stefano Franchi

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C HESS, GAMES, AND FLIES
existence of dominant and recessive traits in particular genes, and by explaining the appear-
ance of visible mutants by their combinations.
However, very soon the Drosophila colonies started to work all too well for the neo-
Mendelian paradigm. Unlike Gregor Mendel’s famous green/yellow peas or Cuénot’s
brown/white/yellow coated mice, Morgan’s fruit flies did not stabilize into a few diverse
variations of the same morphological element. New mutants continued to come up and each
new eye-color (for example) implied that the Mendelian formulas had to be rewritten. Since
the formulas explained the visible characters as an exhaustive combination of basic genes
(later traits), each unexpected mutation sent the geneticists back to the drawing board. 9 This
fact (plus other facts, I am simplifying here) directed Morgan’s group away from neo-Men-
delian explanations toward a different, and more stable, theory of hereditary variation, e.g.
the theory of the gene, and toward the postulation of crossing-over and linkage mechanisms
during chromosome splitting. In turn, the shift from one theory to the other forced another
change in experimental work: neo-Mendelian searches for possible variations were aban-
doned in favor of the technique of genetic mapping that permitted a more stable, quantita-
tive account of mutations capable of almost indefinite expansion (60).
This shift, however, forced a change in the relationship between the insect, the theo-
rists’ and the theory, that now affected directly the fruit fly. In order to obtain the precise
quantitative mapping that the new theory required, the Drosophila stocks had to be liberat-
ed from all the “genetic noise” generated by other kinds of genes naturally occurring in wild
fruit flies. As Kohler puts it, in order to be turned into “a standard experimental instrument
[...] every new type of genetic noise—lethals, suppressors, modifiers—had to be identified
and eliminated, one by one. Drosophila, so to speak, had to be debugged.” 10 The history of
the interaction between genetic theory and Drosophila does not stop here. Kohler shows,
for example, how the standard form of the “constructed” insect that was essential to the suc-
cess of the precise, quantitative genetic mapping turned into a major liability when scien-
9. Kohler, for example, reports Morgan’s desperation when, after having reworked the Mendelian formula
for the third time in two years of intensive labor, yet another eye-color mutant popped up unexpectedly.
See Morgan and Bridges’ s report quoted in Robert Kohler, Lords of the Fly…, 60-61.
10. Robert Kohler, Lords of the Fly…, 66.