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Genetics can be used to characterize biological pathways Complementation tells us if variation is due to mutations in one gene or several genes. Epistasis tells which gene products are involved in common pathways and which act earlier or later in a process. What are the relationships between color types? X purple RR purple Rr white A rr Relationship between 2 chosen color variants Purple is dominant to white A Purple is dominant to White1 purple RR F1 F2 1 RR, X white A rr X purple Rr 2Rr and 1rr Punnet square Male gametes R r Female gametes r R RR Rr Rr rr What are the relationships between color types? X purple RRPP Purple RrPP or RRPp Purple is dominant to red Red rrPP or RRpp Complementation test Cross two recessive mutants to determine if the mutations are in one gene or more than one. X white A rrPP Purple RrPp red rrPP or RRpp Red and white A are caused by mutations in different genes Epistasis Two genes for flower color Are they two steps in the same pathway to make pigment? Where are the two genes in the pathway? 1. Purple is either a mixture of blue and red pigments each made in a separate biochemical pathway. or 2. Purple results from modification of the same precursor from a white precursor to a red intermediate and finally a purple pigment. We can use genetics to distinguish the two possibilities. The effect of variant alleles in multiple genes that affect pigment in combination will answer the question. Pathway 2 Pathway 1 Precursor 1 Precursor 2 R R P Blue Precursor 1 Red Red P Coexpression of blue and red pigment Purple derived from different precursors makes purple. Modification of the same precursor leads to first a red pigment and then a purple pigment Epistasis test Start with complementation test: Cross two recessive mutants to determine if the mutations are in one gene or more than one. X White A rr Purple Rr Pp Red pp Epistasis test part 2 Cross F1 plants from the complementation test And follow how the different alleles segregate in the F2 generation. X Purple F1 Rr Pp Purple F1 Rr Pp ? Punnet Square: two genes with randomly segregating alleles Male gametes Female gametes RP Rp rP rp RP RRPP RRPp RrPP RrPp Rp RRPp RRpp RrPp Rrpp rP RrPP RrPp rrPP rrPp rp RrPp Rrpp rrPp rrpp RrPp X RrPp 9R_P_ 3R_pp 3rrP_ 1rrpp If Pathway 1 Precursor 1 Precursor 2 R P Red Blue Coexpression of Blue and red pigment derived from different precursors Makes purple 9R_P_ 3R_pp Recessive alleles Lead to lack of either Red or blue pigment 3rrP_ 1rrpp Phenotypes: purple red blue white Relationship between white a and red X white A rrPP red RRpp X F1 is all purple RrPp F2 9 3 4 Pathway 2 Modification of the same precursor leads to first a red pigment and then a purple pigment Precursor 1 R rr - get no red precursor neither purple nor red pigment can be made Red P pp – can get red pigment if correct R alleles are present but not purple Purple F2: 9R_P_ Phenotypes: purple 3R_pp 3rrP_ red white 1rrpp white R is epistatic to P Mutations in the R gene cover the effect of mutations in the P gene. This is because R is upstream of P in a biological pathway The P protein requires the wild type function of the R protein. R can be a regulator required to activate expression of P or R can be an enzyme upstream in a biochemical pathway Using multiple allelism tests with diverse recessive mutants, We can identify all the genes specifically involved in making the purple pigment Genetics can be used to determine the order of steps in a biological pathway Epistasis tells which gene products are involved in common pathways and which act earlier or later in a process. Mouse as a model for mammalian genetics Origins of Mouse Genetics Early domestication by Greeks and Romans Chinese and Japanese fondness for unusual-looking mice Early 19th century-popular objects of fancy in Europe Early 20th century-English and American mouse fanciers Early pioneers included LC Dunn, Clarence Little, Sewall Wright, and George Snell Why Mice As an Experimental Organism? Hardy Requires little space Short life cycle Easily bred High fecundity Mammalian species Large amount of phenotypic variation Easy to genetically engineer Evolutionary Relationships Humans Mice Xenopus D. melanogaster C. elegans 1000 900 800 700 600 500 400 300 200 100 0 myr bp A mouse is not a mouse is not a mouse Hundreds of strains Great phenotypic diversity Variation exceeds that in the human population Why is there biological concordance for human and mouse Evolutionary conservation!! genome (gene content, arrangement and sequence) structure (gross and molecular anatomy) function (physiology and molecular circuits) regulatory systems Why is there biological concordance for human and mouse Evolutionary conservation!! Important loci represent a finite set of key regulatory genes “Key” means location in the regulatory network (nodes) Engineered Models Allows controlled experimental testing of • specific genes • specific environmental conditions or exposures Ideally suited to test specific hypothesis generated from human population studies or other laboratory findings Engineered Models Transgenics • usually used to over-express genes • can be global or tissue-specific • can be temporally regulated Knockouts/knockins • usually used in inactivate genes • can be global or tissue-specific • can be temporally regulated • can introduce genes into a foreign locus • can make amino acid modifications UV Mutagenesis in Yeast Geneticists need variation to study the function of gene products. We create variation in the laboratory by mutagenesis Fig. 7.2 Fig. 7.6 Fig. 7.12b1 Fig. 7.12b2 By choosing the correct mutagens, we can control the type of mutations we make Fig. 7.7 Photoreactivation requires photolyase enzyme Mutagenesis of yeast haploid Irradiate with UV. Calculate survival curve Select optimal dose for isolation of mutations. Select on appropriate selective media: Replica plating to identify nutrient deficiencies.