Complementation tests & how to interpret their findings . The reason we screen for mutants is to find genes - you want to find as many mutants as possible to find as many genes as possible, important for the process you're looking at . Why mutagenesis screens are important o In a screen for mutants, you do not make any assumptions about the genes you might find o You do not need to know about the underlying biochemical or physiological systems, or which proteins are involved o You can find many genes from one simple screen and they can all be characterised o Mutants can also be used to find genes with related or interacting functions o Think about these principles in the context of the search for cell cycle genes in yeast · Drosophila eye colour o Wild-type flies have dark red eyes o It is possible to find mutant flies where eye pigment is not dark red o You can find out how many different genes are required for Drosophila to have dark red eyes through a mutagenesis screen o The first step of this process is to identify all the white eyed mutants you can find using a mutagenesis screen o The next step is to cross all the white eyed mutants to wild-type and test whether the mutations are recessive or dominant o You then need to cross all your mutants to each other o This is a complementation test and the results from this will tell you how many different genes you have found o Some of these mutants may have the same phenotype, but represent different genes · Complementation test o The table below shows the phenotype of progeny between all possible combinations of mutants and wild-type. The aim is to determine how many genes have been identified by mutagenesis. o Mutants a-f are all white-eyed, and they are crossed w wild-type, then w each other
wild type/red mutant a mutant b mutant c mutant d mutant e mutant f wild-type red red red red red red mutant a white red red white white red mutant b white red red red white mutant c white red red red mutant/white mutant d white white red mutant e white red mutant f white o We focus first on the first line, where wild-type (wt) is crossed w each of the mutants, w the phenotype of the progeny shown - they all have red eyes o This tells us the mutations are recessive to wild-type bc we see the dominant wt phenotype o This is important bc dominant mutations can't be used in a complementation test. mutant a mutant b mutant c mutant d mutant e mutant f wild-type rec red red red red red o Next, we need to look at the results of each of the mutants crossed against each other (axa, bxb, cxc, etc). we can see they all have white eyes. o From this, we can conclude that when each mutant is crossed