The number of spikelets per rachis node is a key taxonomic character in the tribe Triticeae. In intergeneric F1 plants, including wheat (Triticum) and barley (Hordeum), the single spikelet trait was epistatic, whereas it showed intermediate response in the interspecific hybrids involving the genus Elymus. Further genetic analysis has been hindered by the high sterility of the F1 hybrid plants. In the F1 E. tsukushiensis × H. vulgare, occasional variation in the number of spikelets was seen, apparently due to somatic chromosome instability. This indicates that these aneuploids should provide useful material for further analysis. A series of nullisomics and nulli-tetra compensation lines of Triticum aestivum ssp. vulgare cv. Chinese Spring was observed. From a consideration of all these results, it is proposed that a dosage of six genes on the homoeologous chromosomes of group-2 may suppress the formation of paired spikelets at the rachis nodes at the hexaploid level of wheat.