The barley inflorescence (ear) is composed of a series of triplets, each with one central and two lateral spikelets, arranged along a central rachis. The wild-type triplet consists of a central fertile spikelet, flanked by two sterile spikelets (‘2-row’ barley). Three independent mutations within the HvHOX1 gene underlying the VRS1 locus result in fertile lateral spikelets, conferring the ‘6-row’ phenotype (PMID: 17220272). In addition, the 'deficiens' phenotype, where sterile spikelets are completely absent, has recently been found to be associated with genetic variants within VRS1 (Saisho et al. 2009, Breed Sci, 59:621-628).
The following relevant traits are scored during national varietal registration trials, as described by UPOV/CPVO guidelines:
"Ear: number of rows" (UPOV code 13. Scores@ 1 = 2-row; 2 = 6-row).
"Ear: development of sterile spikelets" (CPVO-TP code 18. Scores: 1 = none/rudimentary (deficiens); 2 = full).
see http://www.upov.int/test_guidelines/en/list.jsp