If the phenotype of yfg does not segregate 1:3 in the mapping cross, then the cross is not suitable for the sort of positional cloning strategy described here, and a different approach must be taken. This is not uncommon, because different ecotypes can modify the phenotype of a given mutant. In these cases, plants that appear to be homozygous yfg may not all be yfg homozygotes, or only plants that carry ecotype-spe-cific alleles at other locations in the genome appear to be homozygous yfg. If positional cloning is attempted using such populations, there is a risk of erroneously identifying plants as yfg homozygotes when they are not homozygous at this locus and/or mapping other loci that are not yfg but affect the phenotype of yfg. In either case, it is very unlikely that yfg will ever be identified successfully.
There are two ways to deal with such a problem. One method is to try crosses to other ecotypes until one is found in which yfg segregates 1:3. Alternatively, as mentioned previously, yfg can be treated as a quantitative trait locus and mapped accordingly. Such an approach, however, is beyond the scope of this manual (see Falconer and Mackay 1996).
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