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Figure 4-4. Evolution of the Hox subfamily of homeobox-containing genes involved in developmental patterning (see Table 7-5 for a description of this family). From an original gene, a small set of "proto-Hox" genes evolved by duplication. From these, subsequent duplication has created chromosomally linked clusters in invertebrates and early vertebrates. These clusters continued to gain and lose genes by tandem duplication and the clusters themselves were duplicated on at least two occasions (perhaps more in fish). Shading indicates likely homologies, that is, genes thought, based on sequence comparisons, to be descended from a specific common ancestor. Gene names are shown for the vertebrate human and Drosophila clusters and for the stem chordate Amphioxus.

terning. Figures of the evolution of the Hox genes are so often published that perhaps everyone is aware of them and they have become trite. But the example is important because the discovery of these genes had a transforming effect on biology. As will be seen in later chapters, not only are these genes used in corresponding structures in very diverse animals, but they are used in a way that was striking when first discovered. The Hox genes persuasively, and dramatically, showed the continuity of animal life and the much greater than expected homologies of structure and process across the animal world.

Hox genes and their action were first identified in patterning of the major fruit fly body axis. That was remarkable enough, as it was one of the first examples of complex patterning to be understood genetically. But then homologous genes were found in vertebrates. In addition, the gene arrangement structure was similar to that in flies, and indeed vertebrates have four separate clusters that resulted from major cluster-duplication events. Comparable sharing has now been extended to most groups of animals, and indeed following these discoveries many more instances of deep conservation of genes have been found, to the extent that it is now perhaps expected rather than surprising. We will see examples in several subsequent chapters.

With some understanding of what is generally found in a molecule of DNA, we can now look briefly at some of its major sequence-based elements that serve to code for various functions. The biology of DNA itself (its replication, packaging, and the like in cells) encompasses core biological traits but is not of primary interest in this book; therefore, we next consider the role of DNA in coding for the production of other substances.

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