The Architecture Of Genetic Regulatory Hierarchies

General features and approach

Many genes can affect the same structure, pattern, or process. In many cases, it is impossible to determine the order of gene action solely by examining mutant phenotypes. Nor is it usually clear whether one gene plays a primary role in a process and other genes play subordinate roles. For example, for any two genes A and B (which based on similar mutant phenotypes appear to control the same process), several regulatory relationships are possible. A could affect the expression or function of B, B could affect A, A and B could affect each other, or A and B may have no effect on each other. With additional genes, one can quickly see that the number of possible regulatory relationships among a set of genes increases exponentially with the size of the set. Given that genetic screens have often identified multiple loci that have similar developmental effects, it is clear that to decipher the temporal order of action and hierarchical relationships between genes, more information is necessary.

Molecular techniques have revealed that the patterns in which major developmental genes are expressed within various body regions, organs, and cells of the developing animal usually correspond to the structures of the larvae or adult whose formation, pattern, or differentiation are affected by mutations in these genes. This understanding has inspired a whole new approach to embryology and the genetic analysis of developmental regulatory mechanisms. Rather than focus on final physical forms, which are often arrested prematurely and disfigured in lethal mutants, the patterns in which key developmental genes are expressed at different developmental stages can be used as surrogates for the ultimate form. Visualization of gene activity in developing animals provides a much more direct and dynamic (that is, both

Developmental Gene

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