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Genetics can be used to characterize biological pathways Epistasis tells which gene products are involved in common pathways and which act earlier or later in a process. Complementation tells us if variation is due to mutations in one gene or several genes.
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What are the relationships between color types? Purple is dominant to white A X purple RR white A rr purple Rr Relationship between 2 chosen color variants
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F2 1 RR, 2Rr and 1rr X Purple is dominant to White1 purple RR white A rr F1 X purple Rr
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Punnet square r r rr Female gametes Male gametes R R RRRr
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What are the relationships between color types? Purple is dominant to red X purple RRPP Purple RrPP or RRPp Red rrPP or RRpp
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Complementation test Red and white A are caused by mutations in different genes X white A rrPP Purple RrPp red rrPP or RRpp Cross two recessive mutants to determine if the mutations are in one gene or more than one.
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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?
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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.
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Precursor 1 Precursor 2 Blue Red Precursor 1 R P Red R P Purple Pathway 1 Pathway 2 Coexpression of blue and red pigment derived from different precursors makes purple. Modification of the same precursor leads to first a red pigment and then a purple pigment
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Epistasis test X White A rr Purple Rr Pp Red pp Start with complementation test: Cross two recessive mutants to determine if the mutations are in one gene or more than one.
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Epistasis test part 2 X Purple F1 Rr Pp Cross F1 plants from the complementation test And follow how the different alleles segregate in the F2 generation. Purple F1 Rr Pp ?
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Punnet Square: two genes with randomly segregating alleles Male gametes Female gametes RP Rp rP rp rPRpRP RRPP RRPp RrPPRrPp RRpp RrPpRrpp RrPPRrPprrPP rrppRrPpRrpprrPp 9R_P_ 3R_pp 3rrP_ 1rrpp RrPp X RrPp
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Precursor 1 Precursor 2 Blue Red R P If Pathway 1 Coexpression of Blue and red pigment derived from different precursors Makes purple 9R_P_3R_pp3rrP_ 1rrpp Phenotypes: purplewhiteredblue Recessive alleles Lead to lack of either Red or blue pigment
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Relationship between white a and red X X white A rrPP red RRpp F1 is all purple RrPp F2 9 43
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F2: 9R_P_ 3R_pp3rrP_ 1rrpp Phenotypes: purplewhiteredwhite rr - get no red precursor neither purple nor red pigment can be made pp – can get red pigment if correct R alleles are present but not purple Precursor 1 Red R P Purple Pathway 2 Modification of the same precursor leads to first a red pigment and then a purple pigment
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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
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Using multiple allelism tests with diverse recessive mutants, We can identify all the genes specifically involved in making the purple pigment
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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.
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Mouse as a model for mammalian genetics
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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
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Why Mice As an Experimental Organism? Short life cycle Easily bred High fecundity Hardy Requires little space Large amount of phenotypic variation Easy to genetically engineer Mammalian species
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Evolutionary Relationships 0 myr bp 1002003004005006007008009001000 C. elegans D. melanogaster Xenopus Mice Humans
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A mouse is not a mouse is not a mouse Hundreds of strains Great phenotypic diversity Variation exceeds that in the human population
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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
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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)
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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
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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
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UV Mutagenesis in Yeast Geneticists need variation to study the function of gene products. We create variation in the laboratory by mutagenesis
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Fig. 7.2
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Fig. 7.6
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Fig. 7.12b1
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Fig. 7.12b2
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By choosing the correct mutagens, we can control the type of mutations we make
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Fig. 7.7
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Photoreactivation requires photolyase enzyme
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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.
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