Are there other lethal gene combinations in Great Danes?

Based on the provided materials, the homozygous Harlequin genotype (HH) is the only genetic combination explicitly identified as lethal in Great Danes.

As previously discussed, the Harlequin mutation is an embryonic lethal gene. When a Great Dane embryo inherits two copies of the Harlequin gene (one from each parent), the HH combination causes the embryo to die and be reabsorbed early in the pregnancy. Because of this fatal outcome, there are no true-breeding harlequins; all living Harlequin Great Danes possess only one copy of the mutation (heterozygous Hh).

It is important to distinguish this embryonic lethality from the other major genetic risk in the breed: the double merle (MM) genotype. While breeding two merle or harlequin dogs together carries a 25% chance of producing double merle puppies with severe congenital health defects—such as blindness, microphthalmia, deafness, and cardiovascular issues—the double merle combination is not lethal. The sources note that despite these severe sensory impairments, many double merles can lead full, well-adjusted lives.

The provided sources do not mention any other lethal genetic combinations affecting the breed.