Catahoula Genetics
Many Catahoulas are blessed with beautiful leopard markings, this is one of the things that make them so unique, however, the merle gene which is responsible for this can also cause havoc from time to time. Basically, to my understanding, there are Catahoulas who are 'non merle', Catahoulas who are 'single merle', and Catahoulas who are 'double merle' , as well as Catahoulas that are what would be called 'cryptic, phantom, or undectectable merles'. There are certain vision and hearing anomalies that may be attributable to a double dose of the merle gene. Fortunately, some dedicated Catahoula-ists are blazing a trail into Catahoula genetic research and there is now a DNA test available for the merle gene.
UPDATE:
Thanks to GenMark for their very nice reply to my email which included this explanation of merle genetics which is the clearest that I have ever seen :
There are three genes associated with merle and our test will identify the two genes that each animal carries. Each animal can carry any combination of the following.
M - Merle
m - wildtype or non-merle
[c] - cryptic merle
The easiest explanation for the difference between the Merle and cryptic gene is the length of the deletion of the merle band. Since this band length can change when the gene is passed on to the offspring, it is possible for it to change from merle to cryptic OR cryptic to merle between the parent and offspring.
Below is the genotype key that will also be on the result report. The genotype represents the "genetic make-up" of the dog and the phenotype refers to the appearance of the dog. As you can see, there are a couple of genotypes that relate to the merle phenotype--you would expect to see merle color pattern on dogs tested as Mm, or MM[c]. You would expect dogs tested as mm, M[c]m, and M[c]M[c] not to express the merle color pattern. MM is double merle and the color pattern on dogs with MM tends to be predominantly white. As noted, dogs with the cryptic merle gene may or may not express the merle color pattern. Sometimes it may only be a small spot somewhere on their body and sometimes you may see an indication of it early in life, but notice that the color pattern becomes masked as the puppy ages and the hair grows in. Being able to identify the presence of a cryptic merle gene is extremely important--without testing it would be difficult, if not impossible, to know whether a dog is carrying this merle gene just by it's appearance.
Merle Genotype Key (genotype : phenotype):
mm - homozygous wildtype : non-merle
Mm - heterozygous merle : merle
MM - homozygous merle : double merle
M[c]m - heterozygous 'cryptic' merle : non-merle
MM[c] - heterozygous 'cryptic' merle : merle
M[c]M[c] - homozygous 'cryptic' merle : non-merle