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Modern Genetic Analysis Chapter 16. The Genetic Basis of Development
Summary

The details of how animal development proceeds will undoubtedly differ from species to species. However, the examples that we have looked at here do portray general themes.

In developmental pathways, regulation is at all possible levels of information transfer between the gene and its active protein product.

Early in life, each cell is totipotent: its lineage has the potential to differentiate into a number of cell types. But, in later stages of development, the type of cell that it can become is increasingly restricted. In other words, the lineage's fate is successively restricted to ever-narrowing possibilities until one and only one fate is specified.

The first step in pattern formation is to establish positional information. In the early embryo, positional information defines the two major body axes: AP and DV. The cytoskeleton plays a key role in establishing positional information and assigning fates through the subcellular localization of specific gene products. The consequences of the positional information are to create a gradient of activity of one or more transcription factors. These transcription factors then initiate a hierarchical decision-making process that takes different cell lineages down different developmental paths, until all necessary fates are specified. Cellcell signaling is crucial to ensuring that all fates are apportioned within a developmental field. Once fates are apportioned, feedback loops ensure that these fate decisions are maintained within a lineage.

Components of developmental pathways are reutilized extensively in the course of development and are highly conserved within the animal kingdom. We therefore surmise that these pathways evolved several hundred million years ago in a common ancestor to all modern animals and, thus, that much of development is thematically comparable in all animals.

© 1999 by W. H. Freeman and Company. All rights reserved.