# 28 - The forward genetic approach to identifying a mutation...

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The forward genetic approach to identifying a mutation and revealing gene interactions Step 1: Create many single-gene purebred homozygous mutant lines (with identical or similar phenotypes) and the appropriate wild type line. Step 2: Test for dominance. Step 3: Test the mutants for allelism - are they at one or several loci? Step 4: Combine the mutants in pairs to form double mutants to see if the genes interact. NOTE: To establish the chromosomal location of the gene and linkage characteristics, you would do two-factor and three-factor crosses, combined with physical mapping techniques.

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Phenotypic ratios in progeny reveal the type of cross
selfing of F 1 x Second filial generation F 2 9 First filial generation F 1 yellow, round yellow, round 3 3 1 : : : Transmission of two traits G/_ ; R/_ G/_ ; r/r g/g ; R/_ g/g ; r/r G/g ; R/r G/g ; R/r

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Computation of probabilities G/g ; W/w x G/g ; W/w F 2 PHENOTYPIC ratio of G:g is 3:1. Ratio of W:w is 3:1. F 1 selfing = G/g x /Gg F 2 = G/G, G/g, G/g, g/g F 2 = 3 yellow: 1 green F 1 selfing = W/w x W/w F 2 = W/W, W/w, W/w, w/w F 2 = 3 round: 1 wrinkled What are two assumptions for these calculations?
What can you conclude from a 9:3:3:1 phenotypic ratio in a dihybrid cross? - two genes are involved - the genes are on different chromosomes - two alleles for each gene, one is dominant the other one recessive - the genes act independently

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Non-Mendelian segregation ratios: two traits - for two unlinked genes the phenotypic ratio in the F 2 is 9:3:3:1 - You can have different ratios with different types of dominance/recessiveness - what happens when you combine a non-Mendelian trait with a Mendelian one?
Non-Mendelian segregation ratios: two traits

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Non-Mendelian segregation ratios: two traits You can use the forked-line method, as you would with only Mendelian traits.
Phenotypes can be variable - Penetrance : the fraction of individuals with a certain genotype that show the phenotype (can be less than 100%) - Expressivity : the degree of expression of the phenotype, can be quite variable and complicates genetic interaction studies

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Penetrance and expressivity normal eye small eye no eye expressivity of the eyeless mutation Pigment intensity
Incomplete penetrance of a dominant allele (makes it very difficult to assess pedigrees)

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• Spring '11
• Sinclair
• Mutation, wild type

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