Imagine that one of the segments in Drosophila is T2, which will form the wing, and that two of the cells in that segment will become the wing disk, and form the entire wing during pupation. Which two cells must this be? Describe them in terms of their gene expression: are they cells that express \"engrailed + hedgehog only\", \"patched only\", or \"patched + wingless only\"? Solution Answer: The effects of a total lack of engrailed function in the wing has revealed three different requirements for engrailed during wing development. 1. It has a late role in patterning the wing margin, which includes cells within the anterior compartment. 2. it is involved in the cell-cell interactions that maintain the expression of the decapentaplegic gene along the antero-posterior compartment boundary. Decapentaplegic, in turn, controls growth through a mechanism that depends on the activity of the gene hedgehog, and the maintenance of the compartment boundary depends on these cell-cell interactions. 3. Finally, there is a global requirement for engrailed in the control of growth through a hedgehog-independent mechanism. Thus, they appears to be cells which express engrailed and hedgehog only..