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4.1 Linkage: basic haploid eukaryotic chromosome mapping

4.1 Linkage: basic haploid eukaryotic chromosome mapping. Haploids organisms. Advantages for genetics studies: There is no dominance or recessivity Only one meiosis in each cross In some fungus and algae, the individual meiosis products stay attached in tetrads Most of them are microbes.

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4.1 Linkage: basic haploid eukaryotic chromosome mapping

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  1. 4.1 Linkage: basic haploid eukaryotic chromosome mapping

  2. Haploids organisms • Advantages for genetics studies: • There is no dominance or recessivity • Only one meiosis in each cross • In some fungus and algae, the individual meiosis products stay attached in tetrads • Most of them are microbes

  3. The linear meiosis of Neurospora

  4. Allele segregation in ordered tetrads in MI • The allele segregation takes place during the first meiotic division (MI)

  5. A second-division segregation pattern in a fungal octad When a crossover between the centromere and the locus takes place, segregation occurs in the second meiotic division (MII)

  6. Four different spindle attachments produce four second-division segregation patterns

  7. Distance locus-centromere • Crossover frequency between the locus and the centromere In this ascus, only half of the chromatids have undergone a crossover

  8. Two loci: a y b • Located in the same chromosome • They are in different arms in the same chromosome • They are in the same arm • One crossover between the centromere and a produces the same MII pattern for the two loci: LINKED LOCI

  9. MI MI MI MI MI MII MII MI MII MII MII MII MII MII a+ b a+ b a b+ a b+ 808 a+ b+ a+ b+ a b a b 1 a+ b+ a+ b a b+ a b 90 a+ b ab a+ b+ a b+ 5 a+ b ab+ a+ b a b+ 90 a+ b+ ab a+ b+ a b 1 a+ b+ ab a+ b a b+ 5 Two loci • Crossing a+ b X a b+. ORDERED ASCI

  10. MI MI MI MI MI MII MII MI MII MII MII MII MII MII a b a+ b a+ b a b+ a b+ 808 a+ b+ a+ b+ a b a b 1 a+ b+ a+ b a b+ a b 90 a+ b ab a+ b+ a b+ 5 a+ b ab+ a+ b a b+ 90 a+ b+ ab a+ b+ a b 1 a+ b+ ab a+ b a b+ 5 a b a b • Three possibilities • Independent Loci : no linkage. Independent segregation. • Most (96) of the a MII asci are also b MII : the third possibility is the correct one. How have the a MII and b MI occurred?

  11. The written order does not mean the ascus order. Unordered asci • Saccharomyces cerevisiae • Unable to locate the centromere • a+ b+ X a b cross. Three possible asci types • a+ b+ • a+ b+ • a b • a b • a+ b • a+ b • a b+ • a b+ • a+ b+ • a+ b • a b+ • a b Parental ditypes no parental ditypes (recombinante) Tetratypes

  12. Maps with unordered asci • What happens when there is no linkage?

  13. a+ b a+ b a b+ a b+ 808 a+ b+ a+ b+ a b a b 1 a+ b+ a+ b a b+ a b 90 a+ b ab a+ b+ a b+ 5 a+ b ab+ a+ b a b+ 90 a+ b+ ab a+ b+ a b 1 a+ b+ ab a+ b a b+ 5 a b 5.2 5.05 10.25/9.3 Two loci in ordered asci • Cross a+ b X a b+. ORDERED ASCI DP DNP TT TT DP DNP TT

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