PowerPoint PresentationSlide 2Slide 3Slide 4Figure 2-22Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Complementation between two recessive black body mutations of DrosophilaSlide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Lecture 6September 8, 2014Extensions of Mendelian GeneticsAllele-allele interactions Gene-gene interactions (epistasis)Interactions of genes in pathwaysGene InteractionsAlleles have a molecular basis at the level of the DNA sequenceBiochemical genetics - Metabolic disordersAllele-allele interactions - One geneDeviations from dominance/recessivenessDistorted segregation ratiosMultiple (pleiotropic) phenotypesGene-Gene interactions - Two genesComplementation test / Allelism test Epistasis (Suppressors)Genetic redundancyA gene is expressed by a two-step processDNATranscriptionmRNATranslationProteinEnzyme Structure RegulationFunction (Molecular phenotype)The DNA sequence of a gene cponsists of protein-coding and non-coding regionsDNATranscribed sequence5' Non-codingprimary transcript (mRNA)3' Non-coding.............ProteinRNAGenee.g. Promoter RegulationIntronExon ExonDNATranscriptionTranslationspliced transcript mRNAProtein coding sequence (CDS)RNASplicingFigure 2-22Effect of mutations on gene function and protein functionAllelesRegulatoryActive siteActive sitePeripheralPeripheralSplicingExample: Mutant sites in the human PAH geneMutant alleles lead to phenylketonuria (PKU, defect in amino acid metabolism)Wild type allele codes for phenylalanine hydroxylase (PAH)PhenylalaninePhenylpyruvic acid (toxic)TyrosinePAHNote: Exons are much shorter than introns•Numerous mutant alleles have been discovered in the human population•Most mutations reside in exons, but some reside in introns•Alleles are strewn throughout the gene sequence*A map of mutant alleles Ex: The Phenylalanine Hydroxylase (PKU) geneMutationsaffecting PAH proteinMutations affectingmRNA splicingAllelic SeriesIntron Exon•Single-base change (SNP)•eliminates a splice acceptor site•causes deletion of the exon 12 from the mRNA causes truncation of the last 50 amino acids from PAH protein •Null allele.•First PAH (PKU) mutation identified (1986)•Deletion•Deletes exon 3 from the mRNA. •Found in Yemenite Jews (q=1/40). •All carriers trace back to one man born in the 17th century (founder effect).•C>G change (SNP)•changes CGG (arginine) to TGG (Trp)•Null allele•Frequent mutation in Eastern and Western European populations, also among French Canadians (founder effect)Heritable defects in phenylalanine metabolismArchibald Garrod, M.D.Proposed in 1902 that Alkaptonuria is due to a recessive allele. A brief history of PKU research1934: Folling (Norway) describes PKU syndrome1947: PKU ascribed to defect in Phe hydroxylase -> Dietary restrictions (casein protein) ameliorate PKU symptoms~~~~: Beadle and Tatum: One-gene-one-enzyme hypothesis.1961: Newborn testing begins (Guthrie)1967: 37 states’ laws require newborn testing1983: Woo identifies human ‘PKU gene’. -> Genetic testing is possible. Biochemical Pathway**PhenylketonuriadA ‘biochemical'model’ of PKUA ‘genetic model’ of phenylketonuria (PKU)Gene InteractionsAlleles have a molecular basis at the level of the DNA sequenceBiochemical genetics - Metabolic disordersAllele-allele interactions - One geneDeviations from dominance/recessivenessDistorted segregation ratiosMultiple (pleiotropic) phenotypesGene-Gene interactions - Two genesComplementation test / Allelism test Epistasis (Suppressors)Genetic redundancyThe reason for dominance/recessivenessTypically, the WT allele is ‘haplosufficient’. I.e. a gene dosage of 1 is enough for normal function.Dominance+/- x +/-+/+ +/- -/-1: 2 : 1purple : whiteGene dosage:2 1 0Deviations from dominance/recessiveness - IA gene dosage of 1 is sufficient.Incomplete dominancee.g. Haploinsufficiency+/- x +/-+/+ +/- -/-1: 2 : 1purple: pink: whiteGene dosage:2 1 0Deviations from dominance/recessiveness - I+/++/--/-Four o’ clock plantIt depends on how you look!Ex: Hemoglobin gene Alleles: HbA (WT); HbSOrganism - HbS is recessive HbA/HbA = healthy HbS/HbA = healthy HbS/HbS = sickle cell anemiaCell - HbS is partially dominant HbA/HbS ‘sickles’ at low O2 * Protein - HbS is codominant with HbA HbA and HbS can both be detectedDNA - Deviations from dominance/recessiveness - II * conditional phenotype: Phenotype only appears under a certain environmental condition. HbS/HbS Red blood cells in sickle cell anemiaMalaria(or is it?)sensitiveresistantresistantCo-dominance of HbS and HbA alleles at the protein levelCodominanceEx: Blood type antigens Alleles: A, B, 0Deviations from dominance/recessiveness - III * A and B are Co-dominant** The O allele is recessive A/0 x B/0A/B A/0 B/0 0/01 : 1 : 1 : 1Phenotype:Blood type AB A B 0 * **Multiple alleles form an allelic series.Example: Eye color in flies Alleles: WT eosin whiteDeviations from dominance/recessiveness – III Bw+ > w-e > w-dominant-------recessivedominant-------recessivefull function partial loss complete loss of function of function'hypomorph' 'null allele'Yellow Agouti (normal) 2 : 1Yellow x YellowEmbryo-lethal1 :Ay/+ Ay/+Ay/ Ay Ay/+ +/+Embryo lethal allele ---> 2:1 ratioDistorted segregation ratiosDeviations from dominance/recessiveness - IVPleiotropic phenotypesDefinition: Genes that affect more than one trait are pleiotropic.Examples: HbS affects anemia and malaria resistance. The Agouti-yellow mutation affects fur color and embryo survival.Gene InteractionsAlleles have a molecular basis at the level of the DNA sequenceBiochemical genetics - Metabolic disordersAllele-allele interactions - One geneDeviations from dominance/recessivenessDistorted segregation ratiosMultiple (pleiotropic) phenotypesGene-Gene interactions - Two genesComplementation test / Allelism test Epistasis (Suppressors)Genetic redundancyIntroduction to Genetic AnalysisGene-Gene interactions - Two genesStandard *Two genes are both needed for the phenotypeComplementation test (allelism test)Two genes contribute differently to the phenotypeEither one of two genes is sufficient (Genetic