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Chapter 4

Chapter 4 Notes- Version2.doc

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Fiona Rawle

Chapter 4Extensions of Mendelian Genetic PrinciplesPrinciple PointsMore than 2 allelic forms of a gene can existany given diploid individual can possess only 2 different alleles of a given genecomplete dominance the same phenotype will result whether the dominant allele is heterozygous or homozygousincomplete dominance phenotype of heterozygote is intermediate between those of 2 homozygotescodominance heterozygote exhibits phenotypes as both homozygotesdifferent genes interact to determine phenotypic characteristicsepistasis modified Mendelian ratios occur bc of gene interactionsphenotypic expression of one gene depends on the genotype of one or more gene locialleles of different genes may be fatal to the individualexistence of such lethal alleles indicates that the product usually produced by the nonlethal allele is essential to the function of the organismsthe gene is called the Essential GenePenetrance is the frequencyat which an allele manifests itself phenotypically within a populationExpressivity is the kinddegree of phenotypic manifestation of a gene or genotype in a particular individualZygotes genetic constitution specifies only the organisms potential to developfunctionorganism develops many things can influence gene expression such as1internal environmentagesex2external environmentnutrition light chemicals temperature infectious agentsvariation in most of genetic traits is determined predominantly by differences in genotypephenotypic differences result from genotypic differencesfor many trait phenotypes are influenced by both genesthe environmentgeneticists found many exceptionsextensions to Mendels principlesDetermining the Number of Genes for Mutations with the Same Phenotypesgeneral genetic approach to studying a biological phenomenon is to isolate mutants with affect that phenomenonmutants are identified by their phenotype and referred to as the mutant phenotypemutant phenotypes are distinct from the wildtype phenotype normal1Complementation Test cistrans testdetermines whether 2 independently isolated mutations with the same phenotype have affected the same or different genesdeveloped by Edward Lewis to study genes in drosophila2 mutations resulting from the same phenotype are crossedthe phenotype of the progeny is observedIf the 2 mutations affect different genes then the progeny will be wildtypemutant heterozygotes for each of the 2 genes involvedIf there is a wildtype copy for each gene the phenotype will be wild type not mutant2 mutants complement each otherIf 2 mutations affect the same gene the progeny will have different mutant version of the gene on each of the 2 homologuesthe phenotype will be mutantComplementation occurs when mutations are in different genesComplementation does not occur when mutations are in the same gene 2 mutants do not complement each otherThis test is done on unknownsExample of Complementation Test on Drosophila2 truebreeding mutant strains have black body colour instead of wildtype grey yellowAll F1flies will have the wildtype body colour Complementation has occurred between mutations in 2 genes each of which is involved in the body colour phenotypeRecessive autosomal gene ebony e when homozygous mutant results in black body colourOn another autosome a different recessive gene black b also results in black body colour when homozygous mutant2 parents are homozygotes ee bbee bb phenotypically blackF1 flies will be genotypically ee bb having wildtype body colour bc there is now one wildtype allele on each gene so complementation has occurredDouble heterozygote was produced by fusion of gametes produced by 2 truebreeding parentsMultiple AllelesAllele that predominates in populations of the organism in the wild is the wildtype allelealternative allele is the mutant allelegiven gene may have several alleles one wildtyperest are mutant not just 2these genes are said to have Multiple Allelesthese genes constitute a multiple allelic seriesa single diploid individual can have only a maximum of 2 of these alleles one on each of the 2 homologous chromosome carrying the gene locusABO Blood Groupexample of multiple alleles of a gene is found in human ABO blood group seriesdiscovered by Karl Landsteiner in early 1900s2
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