Homoeologous sequences and the Wheat GeneChip. The number of probesets with a significant (≥ 7) number of probes matching to 237 homoeologous sequences known to be expressed at some level is shown in this histogram. Categories are labelled as x/y, indicating that x out of 3 homoeologs are represented on the Wheat Genechip by y probesets. For example, "3/1" means that all (or almost all) probes from a single probeset match perfectly to all three homoeologs of a gene and hence that individual contributions from the homoeologs cannot be resolved; "3/2" indicates that the contribution from one homoeolog can be resolved, while the other two hybridize to a single probeset; similarly, "2/1" indicates that two homoeologs match probes from a single probeset, while the third homoeolog does not significantly hybridize to any probes. In only two cases, indicated by "3/3", can the expression of each of the three homoeologs be detected individually by probes from three separate probesets, while in the majority of cases homoeologous triplets (51 out of 56) are significantly matched by probes from only a single probeset.