577 18.1 The logic of building the body plan opment of a pattern includes points at which choices must be made: some are like toggle switches and others are analogous to the preset buttons on a radio. Toggle switches have only two states: ON or OFF. Preset but-tons allow the selection of a single option among many alternative states. All of the molecular elements that constitute choice points have been discussed in earlier chapters. The challenge of pattern formation is to under-stand how these molecular elements are interconnected in a coherent and reproducible way to build the same basic body plan in every member of a species. In genetic circuits, the ultimate effect of these choice points is to modulate the activity of a constella-tion of genes that together give a cell its de±ning devel-opmental characteristics—the kinds of cell types to which it can give rise. A body plan is produced by wiring the gene transcriptional apparatus in a cell in such a way that it communicates its decisions to adjacent cells. In this way, each cell can share with its neighbors infor-mation concerning the status of its developmental deci-sions. As a result of this information sharing, the entire cell population can coordinate a community effort, to ensure that all of the necessary cell types are represented in the proper spatial deployment for building the tissues, organs, and appendages of the mature body plan. Thus,
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