Question

Pattern formation refers to the process by which cells in a developing embryo acquire identities that lead to a well-ordered spatial pattern of cell activities. Do you think the pattern formation would apply to the development of an appendage (e.g. a forelimb)? Explain why or why not

Answer 1

Yes, because we have seen that the segments of the insect larva are all variations on the same basic theme, with segmentation genes defining the basic repetitive module and homeotic selector genes giving each segment its individual character. The same applies to the major appendages of the adult insect body—legs, wings, antennae, mouthparts and external genitalia: they too are variations on a common basic theme. At a finer level of detail, we encounter the same wonderful simplification: the appendages—and many other parts of the body—consist of substructures that are themselves variations on a small number of basic evolutionarily conserved themes.

We follow the course of development in Drosophila through to its end, narrowing our focus at each step to examine one example of the many related structures that are developing in parallel. As we go along, we shall point out parallels with vertebrate structures that develop similarly, using not only the same general strategies, but many of the same specific molecular mechanims. But to avoid interrupting the narrative later, we must first briefly explain some key experimental methods, required to cope with a special problem that arises when we try to discover how genes control the later stages of development.