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Health and Welfare

Breeding

Merling in some breeds of dog is a coat patterning effect acting on the colour pigment eumelanin - it is not a "colour" in its own right. A dog needs to only have one merle gene to show merle patterning, it is not a recessive gene. Double merles - where the dog receives a merle gene from BOTH parents - should be avoided for health reasons

Deafness and blindness are unfortunately common in double merles (MM). For this reason two merles should never be bred together, as this will certainly result in some double merle puppies.

Anyone who breeds merles has to be very careful if they want to avoid double merle puppies. A double merle can only be bred from two merles, as it requires an M gene from each parent. As merle is dominant, no dogs can be carriers of it - any dog with one M gene will be a merle.

BUT - Sometimes the merle gene is hidden, masked by another gene and there is no sign at all that they are merle. A recessive red dog (rr) will not display any merle because it is unable to make eumelanin in its coat, and the merle gene only affects eumelanin (black, liver, blue and isabella (fawn) colour pigment). A clear sable (dog with a red coat due to the sable gene, but no visible black sabling) will also not show any merling because there is no eumelanin to be merled.  In both above cases, a dog may have the dominant gene M but it will not show.

If such a dog is accidently bred to another merle, some of the puppies will be doubles. The genes that code for ear and eye development are closely associated with the M series. Merle causes problems because of lack of pigment in certain vital areas - the eyes and inner ears. Most heterozygous merles have plenty of pigment because they still have an m (non-merle) allele to help them to make it, but double merles often have large white areas where there is no pigment produced at all.