biol139-lecture20-2011

9 three point crosses three point a faster more

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Unformatted text preview: ee-point = 33.9 Three-point crosses Three-point A faster, more accurate method for faster, mapping genes, is through the simultaneous simultaneous analysis of three traits - 3 different genes analysis 3 criteria must be met for a 3-point cross: 1. Genotype of the organism producing the crossover gametes must be heterozygous (trihybrid) 2. Each genotype must be reflected in the phenotype of the resulting offspring 3. Large number of offspring must be obtained to recover a representative sample of recombinants Example of 3-point cross Example • 3 X-linked recessive mutant genes • yellow (y) body colour • white eye (w) colour • echinus eye (ec) shape First Step: • arbitrarily assume the sequence of the three genes assume • i.e. y-w-ec (but could assume w-y-ec) • really doesn’t matter since this is what we are trying to prove (the order) Female: homozygous recessive Female P1 y w y w ec y + w+ X ec Male: hemizygous wild ec+ ywec/ywec y+w+ec+/Y Gametes y w ec y + w+ Female: heterozygous for 3 traits F1 y+ w+ ec+ yw ec ywec/y+w+ec+ X testcross ec+ Males: hemizygous recessive yw ec ywec/Y y + w+ ec+ yw ec F1 Females Gametes 1 2 y w y+ w+ ec ec+ ec 3 y+ w 4 y w+ ec+ 5 yw ec+ 6 y+ w+ ec 7 y w+ ec 8 y+ w ec+ X F2 Male Phenotype y w ec y + w + e c+ y w ec Observed Numbers 4685 y w 4759 y + w+ ec ec+ parentals y w ec y w+ ec+ y w ec y + w+ e c y w ec y + w e c+ 70 Recombinants for y relative to parental 80 combinations w + ec ec 193 Recombinants for ec 207 relative to parental combinations y + w 3 Recombinants for w relative to parental 3 combinations y + ec 10,000 F1 F2 Male Phenotype y + w+ ec+ yw ec Female: heterozygous for 3 traits Observed Numbers y w ec 4685 y y+ w+ ec+ 4759 y + w+ 70 y w+ ec+ 80 193 y+ w+ ec 207 3 y+ w ec+ 3 10,000 w ec ec+ parentals Recombinants for y relative to parental combinations w + ec Recombinants for ec relative to parental combinations y + w Recombinants for w relative to parental combinations y + ec Finding the order of gene linkage and map distances using 3 point crosses 1. Pick any two genes to start with • RF is always a function...
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