Penny, are you claiming there are health issues with Mm?
According to Prevalence of Deafness in Dogs Heterozygous or Homozygous for the Merle Allele the rates of deafness in Mm are 2.7% uni and 0.9% bi. Are these really that different than the rates of deafness in Border Collies or other breeds?
Prevalence of Unilateral and Bilateral Deafness in Border Collies and Association with Phenotype
Hereditary Deafness in Dogs and Cats: Causes, Prevalence, and Current Research
Hi Mark. I really shouldn't be spending time here, as I have work up to my eyeballs. But having checked in...I've seen studies that suggest Mm isn't a problem as well, but then there is this study that indicates otherwise (looking at the percentages in the Reetz study): http://www.pnas.org/...103/5/1376.full (Retrotransposon Insertion in SILV Is Responsible for Merle Patterning of the Domestic Dog) with an excerpt below:
Dogs having Mm and MM genotypes typically have blue eyes and often exhibit a wide range of auditory and ophthalmologic abnormalities (3). Reetz et al. (4) studied the auditory capacity of Dachshunds and found that 54.6% of MM and 36.8% of Mm dogs had auditory dysfunction, ranging from mild to severe deafness. All control dogs (mm) in the study had normal hearing. Klinckmann et al. (5, 6) conducted ophthalmologic studies with three groups of Dachshunds (MM, Mm, and mm) and found that merles and double merles had significantly greater frequencies of ocular abnormalities, including increased intraocular pressure and ametropic eyes. Microphthalmia and colobomas are well described in merle and double merle Dachshunds and Australian Shepherds (3, 7, 8). In all breeds, the double merle genotype can be sublethal and is associated with multiple abnormalities of the skeletal, cardiac, and reproductive systems (3, 9, 10). For these reasons, merle-to-merle breedings are strongly discouraged (9).