FIGURE 7-4 The FLP recombination method for removing DNA sequences from the genome at specific times and places in the animal to inactivate or activate specific genes. This method is now widely used in Drosophila and in mice. (Panel A): A circular piece of DNA is shown, containing two FRT sites in opposite 5′-3′ orientation. When crossover occurs, the gene order ABCD changes to ACBD. (Panel B): Two FRT sites are in the genome in the same orientation. When cross-over occurs, a circle of DNA is looped out carrying the equivalent of one FRT site. The other remains in the genome. See text for details. Source: Golic and Lindquist (1989). Reprinted with permission from Cell Press, copyright 1989.

able in many instances to produce mature eggs lacking the gene product (Perrimon et al. 1989; Chou and Perrimon 1996). This provides the opportunity to determine the phenotype of embryos lacking the function of the gene in question. Similarly, somatic clones can be produced in embryos or larvae heterozygous for various mutations. These somatic clones, induced at high frequency by the FRT-FRP system, will be homozygous for the new mutations and show a phenotype (Xu and Rubin 1993). Thanks to these powerful methods, components of several of the signal transduction pathways have been identified first in Drosophila and C. elegans and later confirmed in vertebrate cell systems.

The Zebrafish

History, Biology, and Genetics

The zebrafish is a relative newcomer to the list of model animals. It has become important as the first vertebrate to be subjected to large-scale genetic screens, which were shown to be feasible by C. Nüsslein-Volhard and others in the 1980s (Haffter and Nüsslein-Volhard 1996). Such screens are possible be-



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