The combination of these models allows the in-depth genetic analysis of many fundamental biological processes using a plethora of different methods, finally providing many specific approaches to combat human diseases. The transparent larvae of zebrafish are uniquely suited to study organ development in a vertebrate, and the superb versatility of reverse genetics in the mouse made it the model organism to study human physiology and diseases. In Drosophila, a rich pattern is generated in the embryo as well as in adults that is used to unravel the underlying mechanisms of morphogenesis. elegans is unbeatable in the analysis of cell-to-cell contacts by saturation mutagenesis, as worms can be grown very fast in very high numbers. Due to their specific strengths, these model organisms have their strongest impacts in rather different areas of biology. Genetic animal model organisms with large research communities are the nematode Caenorhabditis elegans, the fly Drosophila melanogaster, the zebrafish Danio rerio, and the mouse Mus musculus. Later, a focus on a few organisms, which are accessible to systematic genetic investigations, resulted in larger repertoires of methods and applications in these few species. In Darwin’s and Mendel’s times, researchers investigated a wealth of organisms, chosen to solve particular problems for which they seemed especially well suited.

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