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Unformatted text preview: 127 4.3 Linkage maps Deducing gene order by inspection Now that we have had some experience with the three- point testcross, we can look back at the progeny listing and see that for trihybrids of linked genes it is usually possible to deduce gene order by inspection, without a recombinant frequency analysis. Only three gene orders are possible, each with a different gene in the middle po- sition. It is generally true that the double recombinant classes are the smallest ones. Only one order is compati- ble with the smallest classes having been formed by double crossovers, as shown in Figure 4-13; that is, only one order gives double recombinants of genotype v ct cv 1 and v 1 ct 1 cv . Interference Knowing the existence of double crossovers permits us to ask questions about their possible interdependence. We can ask: Are the crossovers in adjacent chromosome regions independent events, or does a crossover in one region affect the likelihood of there being a crossover in an adjacent region? The answer is that generally crossovers inhibit each other in an interaction called in- terference. Double recombinant classes can be used to...
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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.
- Spring '08