Figure 2..

The four-winged fruit fly. In the homeotic mutant bithorax, the halteres, which are a balancing organ, are transformed into a second pair of wings. The same transformation can be achieved by treating fruit fly pupae with ether, and selection of fruit flies by this treatment over several generations for expression of the phenotype in the absence of ether led Waddington to his theory of genetic assimilation [4]. (Figure credit: FlyBase)

Kirschner BMC Biology 2013 11:110   doi:10.1186/1741-7007-11-110
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