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Unformatted text preview: The recombinant genotypes are shown in red. If the genes are linked, a simple approach to mapping their distance apart might be to use the following formula: map distance RF 100(NPD 1/2 T) because this formula gives the percentage of all recom- binants. However, in the 1960s David Perkins developed a formula that compensates for the effects of double crossovers, which are the most common multiple crossovers. Perkins formula thus provides a more accu- rate estimate of map distance: corrected map distance 50(T 6 NPD) We will not go through the derivation of this formula other than to say that it is based on the totals of the PD, T, and NPD classes expected from meioses with 0, 1, and 2 crossovers (it assumes higher numbers are vanish- ingly rare). Lets look at an example of its use. We as- sume that in our hypothetical cross of A B a b , the observed frequencies of the tetrad classes are 0.56 PD, 0.41 T, and 0.03 NPD. Using the Perkins formula, the corrected map distance between the...
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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