Introduction to Genetic Analysis 121

Introduction to Genetic Analysis 121 - 44200_04_p115-150...

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120 Chapter 4 Eukaryote Chromosome Mapping by Recombination Evidence that crossing-over occurs at the four-chromatid stage As previously noted, the diagrammatic representation of crossing-over in Figure 4-3 shows a crossover taking place at the four-chromatid stage of meiosis. However, it was theoretically possible that crossing-over occurred at the two -chromosome stage (before replication). This uncertainty was resolved through the genetic analysis of organisms whose four products of meiosis remain together in groups of four called tetrads. These organ- isms, which we met in Chapter 3, are fungi and unicel- lular algae. The products of meiosis in a single tetrad can be isolated, which is equivalent to isolating all four chromatids from a single meiocyte. Tetrad analyses of crosses in which genes are linked show many tetrads that contain four different allele combinations. For ex- ample, from the cross AB ± ab some (but not all) tetrads contain four genotypes: AB Ab aB ab This result can be explained only if crossovers occur
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.

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