Why are Harlequin-to-Harlequin matings considered high risk?

Harlequin-to-Harlequin matings are considered high risk for two primary genetic reasons: the creation of double merles with severe health defects and the occurrence of embryonic lethality.

• The Risk of “Double Merle” Health Defects: Because the Harlequin gene requires the Merle gene to be visually expressed, every Harlequin Great Dane carries at least one copy of the Merle gene. Therefore, breeding two Harlequins together is genetically equivalent to a Merle-to-Merle mating. This pairing carries a 25% probability of producing “double merle” puppies, which inherit a homozygous MM genotype. Double merles are predominantly white and suffer from a high incidence of congenital health issues, most notably unilateral or bilateral deafness, as well as severe ocular problems like microphthalmia (abnormally small, malformed eyes).

• Embryonic Lethality and Smaller Litters: Because possessing two copies of the Harlequin gene (HH) is lethal, all living Harlequin dogs are heterozygous (Hh). When two Harlequins are bred, there is a 25% chance that an embryo will inherit the homozygous HH genotype. This mutation causes the embryo to die and be reabsorbed early in the pregnancy. Because these puppies fail to form, Harlequin-to-Harlequin matings tend to produce noticeably smaller litters.

Due to the severe sensory impairments and health risks associated with producing double merle puppies, experts and rescue groups strongly discourage Harlequin-to-Harlequin matings. In some countries, breeding two Harlequin dogs together is even illegal.