Epistasis

As the Bombay phenotype demonstrates, it is actually proteins, not the genes, that interact. After identifying interacting loci, the next step is determining the proteins that the genes at those loci encode, and the properties of those proteins.

The emerging field that involves the study of proteins and protein interactions is called proteomics. New techniques are now available to locate proteins that interact with one another. In the yeast two-hybrid system, one such technique, one protein is used as bait, and a pool of unknown proteins, referred to as prey proteins, are tested to see if any of them bind to the bait. Binding, if it occurs, triggers a reaction that causes yeast cells to turn blue. In one experiment testing a protein's interactions with more than 1,000 other proteins, 950 interactions were found. Not all of these interactions are likely to occur or be important in the organism, but such results indicate how common, and complex, protein interactions are in living organisms.