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Lecture 9

Notes Lecture 9.odt

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McGill University
Biology (Sci)
BIOL 202
Daniel Schoen

Note: there was an error in the chi squared example on the slides Lecture 9: phenotypes and gene interactions, part 2 • Complementation testing (testing for allelism) • types of interactions among loci • penetrance and expressivity • wrap up Recall: mutations = tools to understand biological functions (scalpels) easiest way to go on a mutant hunt is to use a haploid organism (ie: neurospora, even though it doesnt have a lot of visible phenotypes) Beedle and Tatum used X rays (ionizing radiation can cause mutations) to speed up metabolism mutations in neurospora Interactions between genes in pathways conidia wild type are mated with wild type of opposite mating type inoculate multiple samples of complete medium with single conidia spores (need to rescue anything that can potentially be a mutant....if you went directly to minimal medium, they would die and you'd lose your mutant) Then test them on minimal media (has minimal amount of nutrients that the wild type needs to survive on), those without growth indicate a nutritional mutant (want to rule out potentially sick neurosporas, because sickness isn't necessarily genetic....something that segregates 1:1 in an octad is a mutation, not a sickness) Test the nutritional mutants on supplemented minimal media (ex: minimal media+ amino acids) Figured that the amino acid they were lacking was R (Arg) Arginine has 2 intermediates, ornithine and citrulline.... See chart in slides for the pattern The wild type has all 3 enzymes required to make precursor -->ornithine--->citrulline-->arginine a biosynthetic pathway in studying gene-gene interactions... 1. often faced wt a series of mutations tht influence the same phenotype 2. These mutations could be alleles of the same or different loci, we don't know in advance, usually 3. First determine whether our mutations are alleles of the same or several different loci. For recessive mutations, use COMPLEMENTATION TEST to answer this question 4. if we have determined that there are 2 or more mutations at different loci, we are ina position to study the nature of this interaction between the genes ex: Interactions between genes in pathways...true breeding white flowers....are these mutations of the same or different alleles Complementation testing when working with diploids, need to work wt mutations that are recessive and need true breeding lines First inbreed the white plants, get true breeding white plants, cross back with dominant genotype (homozygous) look for 3:1 phenotype ratio to make sure you actually have a true breeding white flower. if you can cross 2 recessive mutants and receive a dominant phenotype, than you have complementation, and your mutations are on different genes (you still have one working copy of each ge
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