FIGURE 9-2 Phenotypes of mutations in genes encoding transcription factors. (a) A dominant mutation in the human HOXD13 gene results in the development of extra digits, a condition known as polydactyly. (b) Homozygous recessive mutations that prevent expression of the Ubx gene in the third thoracic segment of Drosophila result in transformation of that segment, which normally has a balancing organ called a haltere, into a second copy of the thoracic segment that develops wings. (c) Mutations in Arabidopsis thaliana that inactivate both copies of three floral organ–identity genes transform the normal parts of the flower into leaflike structures. In each case, these mutations affect master regulatory transcription factors that regulate multiple genes, including many genes encoding other transcription factors.

[Part (a), left, Lightvision, LLC/Moment Open/Getty Images; right, Goodman, F. R. and Scrambler, P. J., Human HOX gene mutations. Clinical Genetics, 2001, 59:1, pages 1–11. Part (b) from “The bithorax complex: the first fifty years,” by Edward B. Lewis, reproduced with permission from The International Journal of Developmental Biology, 1998, Vol 42(403-15), Figures 4a and 4b. Part (c) republished with permission of Elsevier, from Weigel, D. and Meyerowitz, M., “The ABCs of floral homeotic genes,” Cell, 1994, 78(2):203-209; permission conveyed through Copyright Clearance Center, Inc.]