The Harlequin coat colour pattern is a white coat colour with irregular patches of black or other dark colour.
The Harlequin gene does not cause the Harlequin pattern without the merle gene.
A Harlquin Great Dane must have both the Harlequin gene and the Merle gene.
Harlequin is a merle modifier, it turns the areas between the dark patches into white, this is why Harlequin is only visible on dogs with the merle gene.
Harlequin will not affect the coat colour of dogs that don't have the merle gene. Dogs without the merle gene can have the Harlequin gene but without expressing it.
If the dog is double merle (M/M) the Harlequin gene effect may not be noticeable.
The harlequin gene is dominant which means that only one copy of the gene is enough to express the Harlequin pattern in merle dogs.
Dogs with two copies of the Harlequin gene H/H will very likley die during gestation and will be reabsorbed into the womb, therefore breeding Harlequin (H/h) to Harlequin (H/h) may result in smaller litter size.
h/h (No Harlequin): the dog does not carry the Harlequin gene, the dog will pass the h allele to its entire offspring.
H/h (Carrier): The dog carries one copy of the dominant Harlequin allele H which results in the expression of the Harlequin pattern only in dogs with one (M/m) or two (M/M) copies of the merle allele at the M locus.(Please note that Harlequin expression in M/M dogs at the M locus may not apparent)
H/H: Embryonic Lethal, dogs with H/H die before birth.
Please note that the merle pattern is not expressed in dogs with Pheomelanin coat colours (Red, fawn, Sable, Cream) so you cannot tell if the dog carries the merle gene or not, However, if the dog is H/h, the harlequin pattern will be expressed, this is known as "fawnequin".