Breeding data from Eire and Great Britain confirm the hypothesis of Sponenberg (1985) that the harlequin colour of the Great Dane breed of dog is due the combined action of a dominant gene H with the merle gene M in the genotype H+M+. The typical bluish coloration induced by M is modified to white by the action of H. The H gene is a prenatal lethal when homozygous HH and this study offers clear indication that the heterozygous H+ interacts with M to reduce the viability of white merle homozygotes H+MM.