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Chapter 4a-2.docx

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BIOL 2000
Joseph Rasmussen

Chapter 4  Linked alleles tend to be inherited together  Morgan suggested that crossing over produces allelic combinations that are different from the meiotic input (recombinants)  Morgan suggested that chromosomes break and re-join during pairing, and that chiasmata are visual evidence of that event. Note  If we do not know that genes are linked separate gene pairs with “.” o pr/pr+ . vg/vg+  If we know genes are independently assorting (unlinked), separate gene pairs with “;” o a/a+ ; b/b+ Crossing over occurs during prophase  Morgan proposed that crossing over between chromatids, not chromosomes  A second division segregation pattern in a fungal octad demonstrates that crossing over must be between chromatids  Multiple crossovers can include more than two chromatids to generate multiple types of linear tetrads o Centrosome decides where the crossing over occurs o Can occur on both sides of centromere  SUMMARY: Crossovers generate recombinant meiotic products Mapping by recombinant frequency  Morgan’s result: if test crosses were done involving different pairs of genes, saw different ratios of parental: recombinant  Morgan’s hypothesis: frequency of recombinant products may be related to distance between genes  Alfred Sturtevant: If Morgan’s hypothesis is true, should be bale to use recombinant frequency to ree
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