One of the things budding geneticists, biochemists and cell biologists learn very quickly when they enter grad school is that studying humans is usually not the best way to successfully tackle the most interesting research questions. You can ask questions about human biology, but to answer them you generally turn to an elite club sometimes called the Security Council of biology: the bedrock group of five model organisms.
Fruit flies, baker's yeast, roundworms, E. coli, and mice make up the core membership this elite club - the vast majority of major breakthroughs in modern biology has come from work on these organisms. You can also include a few other organisms like the mustard weed, zebrafish, and the frog Xenopus laevis (actually, mainly frog eggs), but model organisms aren't just picked at random, and potential new members go through a long vetting process. It takes a significant investment of time and resources to turn a wild organism in to an experimentally tractable system, so scientists generally try to get the most mileage out of the model organisms we already have.
If you're interested in understanding more about the crucial role these critters play in biology research, check out this review (free PDF here).