The reasons for using animals for modern genetics research are several fold, and these different organisms all have different characteristics that are taken advantage of:

Bacteria: It was discovered that bacteria could carry circular DNAs that were replicated in the bacterium itself. Therefore, we use bacteria to move DNA of interest around AND to make MORE of it, so that the DNA itself is not limiting. Further, it was found that we could manipulate the bacteria so that on the DNA was a selectable marker for an antibiotic, so then you could SELECT specific bacterial cells that contain the DNA of interest. These circular DNAs are called vectors.

Advances were then discovered that allowed using the bacteria to over-produce the protein encoded by the DNA. As you know, the central dogma states that DNA encodes messenger RNA (mRNA) and mRNA is translated into proteins. This technique made CLONING single genes possible.

Drosophila (fruit flies) are used for classical genetic experiments where you can remove small sections of DNA from their chromosomes, and then look at the resulting embryos to see what the effect of removing that section of DNA was. This technique has been critical to our understanding of how most proteins function during development of a multi-cellular animal. Most interesting is the fact that the Drosophila genome is so similar to ours, so once a gene (contained in the DNA that was removed) has been found, that gene is then characterized in vertebrate systems. Also, Drosophila have been used to help understand how maternal factors (proteins and mRNAs from the mother) control cell fate such that a cell becomes the right type of cell (endoderm, mesoderm, ectoderm) then to organ specification....epidermis, sensory, brain, etc.

Mice: Mice have been used to make "knock outs" of genes. Once the gene is knocked out, then the protein is not expressed, and we look at how that effects vertebrate development. Mice are needed for these experiments because as a whole animal, they are closer to humans than fruit flies (they are vertebrates). Also, the technology was developed to make genetically engineered mice that taking normal genes out and replacing them with faulty, defective ones was possible. After this removal and transfection of DNA in a female mouse, you wait to see what effects (usually major defects in development of the animal) occur.

Web page of interest:fruit flies in relation to human disease:

fruit flies