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forward_genetics - BICD100 Fall 2005 Forward Genetics...

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BICD100 Fall 2005 Reinagel Forward Genetics Topics Concepts of forward genetics, applicable to any type of organism we have studied in this class (phage, bacteria, yeast, fruit flies, plants, and hypothetical beasts). 1) "isolating a mutant" The first step in forward genetics is deciding what your looking for. What process do you want to study (sugar metabolism, embryogenesis, pigment pattern formation)? Using a mutagen to increase the probability of mutations one then looks through the population of resulting progeny for individuals showing the phenotype of interest (can’t grow on glucose, failure to gastrulate, having spots instead of stripes). Every individual found (isolated) represents a different independent change in the nucleotide sequence of a gene (mutation) and thus is a mutant. 2) difference between selection and screen The easiest experimental design to carryout when looking for mutants is a selection. In a selection the progeny of the mutagenized individuals are placed in a condition in which the starting strain would die and mutations in genes of a given process will live. The mutants will have a selective advantage. It is often hard to establish a condition that will give the mutants a strong selective advantage over the starting strain. In this case one must do a screen. In a screen one must look through all of the living progeny and find the ones with the phenotype of interest. This approach is more labor intensive, having to deal with all of the normal individuals, but allows a patient person the ability to look at processes that would likely never be able to be studied by selections. These processes would include find the genes required for embryogenesis as mutations in these genes are themselves lethal. 3) two mutants with the same phenotype: same or different gene? Mutants that are isolated are independent changes however they can be changes in the same gene or in different genes. There are two ways to determine if mutants have changes in the same genes or in different genes: complementation and segregation. a) do they complement? To be able to use complementation to test if two mutations are in the same gene these mutations must meet some criteria: Each must be recessive mutations The phenotype in each strain must be the result of a single mutation If the mutants being tested meet these criteria then a complementation test can be used.
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