|These mice are from the inbred strain BALB/c, one of the most commonly used inbred strains in experimentation|
Inbred strains are lines of animals bred to be genetically identical. This is done for experimental purposes, as experiments require the elimination of as many unknown variables as possible. This is to eliminate the possibility that the experimental result was caused by a variable that is not being tested for. For example, genetic variation may cause a particular mouse (or rat or guinea pig) to be more resistant to toxin A than another mouse, so using genetically identical mice will eliminate that possibility. This is also why other variables, such as type and amount of food, size of cage, bedding, and other traits not pertaining to the variable being tested should be strictly controlled.
So, how do you go about creating an inbred strain? By inbreeding, of course! However, it takes some time. The process involves brother-sister matings for at least twenty generations. At that point, the inbreeding coefficient is 99% or higher, and as such the resulting individuals are basically genetically identical. Each inbred strain has been given a designation of numbers and/or letters (such as the mice strains BALB/c, C17, MOM, T739, and perhaps the most often used C57BL), and different strains are used for different experiments. This is often due to a particular trait that is known to occur in the strain under certain conditions. There are many strains that have been selected for certain traits for this exact reason.
Though many people are steadfastly against the use of live animal experimentation, you cannot deny how much has been learned. If it weren't for live animal experimentation, we wouldn't know very much about cancer, behavior modification, or numerous other aspects of modern science. Inbred strains continue to be used so that more can be found out about these and other topics.
My first knowledge of inbred strains came from my Psychology course, which was taught by an animal behavior specialist. He often worked with inbred strains, as did most (if not all) of the Psychology majors on the campus. However, they usually worked with rats (such as strain F344).
Source is Rutgers. Image is from Wikimedia Commons under a Creative Commons license.