Fig. 2. The additivity of segment-specific functions. Diagrams of two Drosophila larvae, anterior to the top. (A) A wild-type larva; (B) a larva mutant for the bxd/pbx segment-specific function. The diagram next to each larva represents the presence or absence of the segment-specific functions that are required to determine a particular segment/parasegment (moving across them horizontally). Underneath these are the three BX-C homeotic genes, Ubx, abd-A and Abd-B. In embryos that lack the entire BX-C, all segments posterior to the second thoracic segment (T2) develop as T2; thus T2 represents the ground state in this model. Because mutations in individual segment-specific functions always cause homeotic transformations towards the last unaffected, more-anterior segment, Ed Lewis proposed that segment-specific functions act in an additive fashion (Lewis, 1978). (A) The wild-type larva shows the segment-specific functions required for the proper development of each segment/parasegment. (B) The mutant larva lacks the bxd/pbx function and therefore has its A1 segment transformed into a copy of T3. Note that ventral pits (a characteristic of T3; arrows) are present in all of the more-posterior segments, indicating that the bxd/pbx segment-specific functions are also required in more-posterior segments (asterisks).