FIGURE 3.

The proportion of major genes expected to be uncovered in a transgenomic screen. Based on evolutionary principles we would predict that 50% of the major genes responsible for a phenotypic difference between the donor and recipient species will be found in a unidirectional transgenomic screen. To see why this is so, imagine a potential donor and recipient species for a transgenomic screen that differ in a phenotype (circle vs. square, respectively). Without further information it is equally likely that: (A) the donor species has the derived phenotype, with a change having occurred on the lineage from the common ancestor to the donor species (upper panels), or (B) the donor has the ancestral phenotype, with a change having occurred on the lineage from the common ancestor to the recipient species (lower panels). The mutation that gave rise to the derived phenotype could have been fully recessive or at least partially dominant. If the donor has a derived phenotype that is dominant (top right), or it has an ancestral phenotype that is dominant (bottom right), then moving the causal gene into the recipient will yield a dominant phenotype. In approximately 50% of cases (left panels) the causal gene will not yield a dominant phenotype when moved from the donor to the recipient. The major genes missed in a unidirectional transgenomic screen could theoretically be found with a reciprocal screen in which the genome of the former recipient species is screened in the background of the former donor species.