For example, we can look at the genetic differences in mice which determine the numbers of platelets in their blood. We study mice within a pedigree sharing a mutation, then work out which parent donated what part of the genome, check the relevant DNA region using a genetic database and then put that gene back into the mouse to produce the selected number of platelets. Rather than using trial and error, we can now work on the genes we are interested in from the start. For drug discovery, it is important to find targets. Rather than using a normal mouse, we can start with a mouse with disease.
Most mutations are deleterious, reducing protein function.
Drugs do the same thing. Fat yellow mice are a model for obesity. We can now generate random mutations in fat yellow mice and find one that causes an offspring to be thin and black. The essential step is to find what was different about its genetic make-up and this can be achieved much more easily now. Once identified, the gene that eliminates a disease is likely to be a good target for a drug.