I'm having a hard time understanding exactly what is involved in functional heterologous complementation, can someone explain the procedure?  I know that the main point is to identify genes that are responsible for specific functions and restore mutants to wild-type for certain features, but I'm having a hard time "getting" exactly what is involved in the procedure.

Thanks the idea is simple –you functionally rescue a genetic deficit (be it colour, an enzymic activity or indeed any function that is lost as a result of a genetic defect). Once you have identified the clone containing the gene that rescues the deficit you can sequence it and thus work back to the cDNA. In some cases the answer is obvious ie you add back in the wildtype (or very similar) gene that “rescues” the one that is mutated. What is often far more interesting is the discovery that an apparently unrelated gene will work just as well as the mutated one and eventually it becomes clear that one has identified a whole signalling cascade and a gene lower down the pathway can rescue the deficit if it is up-stream.

If the above sounds complex to do it isn’t actually in tractable organisms eg yeast, e-coli or even sometimes flies and worms. This has even been done in human cells in culture but is much rarer because of the technical complexity. The link below is a quite simple animated schemata that you might find helpful.

http://www.biostudio.com/d_%20Cloning%2 … tation.htm