The 2-/6-row phenotype is predominantly controlled by the VRS1 locus on chromosome 2H, with the wild type Vrs1.b alleles conferring the wild-type 2-row phenotype, and mutant VRS1.a alleles conferring the 6-row phenotype. However, mutation studies have shown that the phenotypic effect of the wild-type 2-row Vrs1.b allele can be modified by mutations at a second locus, INT-C, located on chromosome 4H. Wild-type Int-C.b alleles are associated with Vrs1.b alleles, conferring the standard 2-row phenotype, in which the sterile spikelets are sterile. However, when Int-c.a alleles are combined with Vrs1.b alleles, this results in enlarged partially sterile lateral spikelets. Similarly, when Int-C.b alleles are combined with Vrs1.a alleles, smaller lateral lateral spikelets are formed.

While the underlying causative mutation underlying the naturally occuring Int-C.a allele is unknown, haplotypes at two SNPs predict allelic state at INT-C, termed here HvINTC_C124G and HvINTC_C498T. Haplotypes CT and CC are diagnostic for the Int-C.b allele. Haplotype GC is diagnostic fot the Int-C.a allele.

PMID: 21217754