Explanation of the HIS4 TRP1 example in Monday s 2 5 07 lecture What if two crossovers T Slaby HIS trp HIS X trp X his TRP his TRP HIS trp his TRP HIS trp his TRP HIS trp 2 X overs his TRP On diff chromos HIS trp his TRP Meiosis I homologs Separate HIS trp his TRP HIS trp his TRP Possible combos of these alleles parentals nonparentals HIS trp OR HIS TRP his TRP his trp Meiosis II Sisters Separate parentals HIS trp his TRP OR nonparentals HIS TRP his trp Pairing each of these mutually exclusive possibilities results in NPD HIS TRP his trp HIS TRP his trp Ratios 1 TT HIS trp HIS TRP his TRP his trp 2 PD HIS trp HIS trp his TRP his TRP 1 Recombination event Tetrad pattern that results tween CENx and his4 only 100 chance of Tetratype TT tween CENx and his4 And tween CENy and trp1 50 chance of a TT 25 chance of a NPD 25 chance of a PD If you dissect 100 tetrads and all only recombined between CENx and his4 and not CENy and trp1 then you d have 100 Tetratypes or 200 recombinants 100x2 200 If you dissect 100 tetrads and all had both recombination events then you d have 50 TT 25 NPD and 25 PD or 200 recombinants 50x2 25x4 200 Summary and Question s for the Students Thus for the purpose of determining if two genes are linked it makes no difference how far away each is from its chromosome s centromere If you wanted to estimate how far away the his4 locus is from its chromosome s centromere Would it matter if trp1 is tightly linked to its chromosome s centromere How so hint trp1 is tightly linked to its centromere Why would researchers find using Neurospora easier than S cerevisiae baker s yeast for mapping mutations relative to a centromere
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