The importance of the octad and tetrad segregations that we discussed in the last chapter extends far beyond their contribution to the mapping of chromosomes. Their study reveals phenomena that depend on the molecular details of the process of recombination. Tetrad analysis enabled us to start understanding recombination, and only fungal octads were able to do this. Examination of large numbers of asci segregating a single gene difference (e.g.,black/white spores) will certainly reveal that the majority fall into the categories we illustrated in Fig.5.3, but there will always be a minority that, instead of showing the usual 4:4 segregation of the two alleles of the gene, have ratios of either 6:2 (e.g., 6 black spores to 2 white spores in the same octad) or 2:6 (i.e., 2 black spores to 6 white spores). These are called aberrant segregations because they seem to imply deviation from the Mendelian expectation that alleles segregate in equal numbers at meiosis.
KeywordsHomologous Recombination Gene Conversion Synaptonemal Complex Strand Exchange Holliday Junction
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