|Deafness and Color-Related Eye Defects in White Great Danes|
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The Great Dane has a broad range of coat colors and the standard, acceptable coat colors include: Black, Blue, Brindle, Fawn, Harlequin, and Mantle. Various non-standard colors do occur in this breed and include some variations involving the merle gene that can result in defects of hearing and sight. In breeding Harlequin and Mantle Danes, whites and merles may result. Danes predominately white are typically deaf and may have various eye anomalies. This brief article attempts to explain why these non-standard colors do occur and why these defects occur, as well as why the defects associated with white Danes may not be passed on directly to their progeny.
When breeding any two merle-bearing Danes (Harlequins, Merles, Whites), some of the offspring can be expected to receive two copies of the merle gene and thus be defined as white Danes. Danes predominately white are typically deaf and may have various eye anomalies. Such dogs can occur from breeding Harlequins to merles or whites (or merles or whites to each other). When breeding Mantle or Black Danes to Harlequins, these merle whites cannot be produced, so mating Harlequins to Blacks or Mantles significantly reduces the risk of producing Danes with the sensory defects associated with the merle gene.
The merle gene that produces these issues is a required part of the Harlequin genotype. But the merle gene when homozygous generally suppresses pigment production by melanocyte (pigment producing cells in the skin and associated hair follicles). Pigment produced by melanocytes also plays an important role in normal hearing and normal sight. White Danes typically result from the presence of two copies of the merle gene (a homozygote dominant for merle).
White Great Danes frequently have little or no pigment in the middle ear thus causing deafness. A change in the number of mature melanocytes located in the hair follicles or the composition of pigment-loaded melanosomes will affect the color in the growing hair shaft. Within the middle ear sound waves are translated into nerve impulses, which the brain in turn interprets as sound. This is done by sound waves at the eardrum causing movement in the fluid within the middle ear. Small hairs, which sit in a bed of pigment, detect this movement. As the hairs move they move the pigment, which in turn, is detected by the nerves. The effects of the merle gene can also result in a variety of eye abnormalities, including congenital cataracts associated with microphthalmia, enophthalmia, medial canthal syndrome, colombomata, tapetal hypoplasia, and heterochromia iridis/hypochromia iridis. This syndrome is collectively called merle ocular dysgenesis.
Since the merle gene has an unusual characteristic called somatic cell mutation, where some cells have reverted to the normal recessive form of the gene, homozygous merle dogs can therefore produce some pigment. So it is common to see some spotting on the heads and rumps of white Great Danes and this is the result of this somatic cell mutation. When this occurs within the middle ear, it can restore some hearing and provide for normal eyesight and eye structure in some white Great Danes. Not all white Great Danes will be noticeably deaf or necessarily have sight issues, but the majority of white Danes will be affected.
Merle deafness, the deafness typically seen in white Great Danes, is almost never directly passed onto their merle or harlequin offspring (only to other white Danes). Piebald deafness is not typically our breed issue when deafness occurs, but it is theoretically possible that piebaldism combined with other spotting genes like harlequin and merle, can result in an increased potential for congenital sensorial deafness. It is useful to test predominantely white Danes for normal hearing (BAER testing) and normal sight (CERF testing); this way both the breed and the breeder can benefit by learning more about the health issues related to merling and loss of pigment.
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