Methods for studying various things like disease, cellular protein expression, and various molecular biology/cell biology related issues.
These can be generally be divided into Classical genetics and molecular genetics.
A classical method would be to look at the family history of the life form you are studying and relate that to known facts. An example of this might be the study of a genetic disease like Lesch Nyhan's disease and the recognition that only men have this disease and therefore it is "sex-linked" (more specifically it is a result of genetic mutation of the Y chromosome (females, which are XX, do not have the Y chromosome)).
Molecular genetics, however, generally involves gene knockout or gene replacement (the addition, change, or removal of a part of the genetic material the codes for a protein). For example, transgenic mice that have had their Myosin 5 encoding gene knocked out, appear almost completely without pigment, because this protein is needed to carry and process pigment in vacuoles within cells. Gene insertion is also possible (and will hopefully be used more extensively in an emerging medical technology called gene therapy). A common example of present gene insertion is the addition of DNA that encodes for GFP (green flourescent protein, so called because it glows green). This protein is often added as a "marker protein" (also called a protein tag, see gene splicing)) along with a more interesting, but less easily detectable protein. (Gene's are generally inserted via "transfection", use of a viral vector, or microinjection into the nucleus of the cell.)
Cell biologists, medical researchers, molecular biologists, and other scientists are generally interested in using one of three organisms with genetic techniques: yeast, transgenic mice, and humans. Each of these have distinct advantages and disadvantages. Yeast are cheap, great for classical genetics (rapidly reproducing), and easily used/abused with molecular genetics, but unfortunately are not great "animal models" of how things might work in people. Mice are better animal models, pretty decent for classical genetics (though MUCH slower to reproduce), and very useful in molecular genetics, but they are also rather expensive. Humans of course make excellent animal models, but are horrible for molecular genetics because everything is unethical, and for similar reasons, less useful than animals that you can drug, cut open, and cross with careful predetermined precision for classical genetics techniques.