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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Forming Complex Pattern: Utilizing Positional Information to Establish Cell Fates Figure 16-24. The homeotic transformation of the third thoracic segment (T3) of Drosophila into an extra second thoracic segment (T2). (a) Diagram showing the normal thoracic and abdominal segments; note the rudimentary wing structure normally derived from T3. Most of the thorax of the fly, including the wings and the dorsal part of the thorax, comes from T2. (b) A wild-type fly with one copy of T2 and one of T3. (c) A bithorax triple mutant homozygote completely transforms T3 into a second copy of T2. Note the second dorsal thorax and second pair of wings (T2 structures) and the absence of the halteres (T3 structures). (From E. B. Lewis, Nature 276, 1978, 565. Photographs courtesy of E. B. Lewis. Reprinted by permission of Nature. Copyright © 1978 by Macmillan Journals Ltd.)

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