The Australian Shepherd is a breed that has been constructed to maximize two different lethal semi-dominant traits: Merle and Bobtail. Without the careful hand of breeders to prevent Merle x Merle breedings and perhaps even Bobtail x Bobtail breedings, what would the expected outcome be of a wild population of Aussies with both of these traits saturated in the founding population?
Let’s pretend that the prophecies of doom in 2012 are true and that human civilization fails, leaving herds of wild Australian Shepherds roaming the West. Aussies are elitists and will not breed with other canids, including other dog breeds, but they are progressive enough not to stigmatize Merle and Bobtail. What would that population look like?
As we learned before, when you have a lethal semi-dominant trait where the homozygous form doesn’t produce viable offspring, you can never breed true. A certain percent of your puppies will always retain the wildtype features: in this case long tails and solid patterned coats. I wouldn’t be surprised if the only reason that solid coats are still allowed in the Australian Shepherd breed standard is because it’s impossible to breed it out.
Aussie graphics provided courtesy of Cartoonize My Pet.
As you can see from the chart above, out of 16 possible outcomes, only 9 produce viable offspring: a 43% failure rate. Of the 9, less than half of them are born ideal copies of their parents (the 4 along the / diagonal) and the rest are suboptimal in some manner. Two of the remainder lack a tail but are the boring and undesirable solid coloring, and three of the rest are born with tails, requiring manual bobbing anyway. One dog has neither the right coat color or the right tail, and could thus be confused for a Border Collie, perhaps the most aggravating result of all to the fancy.
I’ve included the tri-color feature in both parents as well to demonstrate the key difference between semi-dominant and either dominant or recessive traits. Tricolor is a simple recessive and if both parents are tricolor all their offspring will be tricolor too. Likewise, if both parents were homozygous for a dominant trait, all their offspring would display that trait as well and all will pass it along.
Things aren’t looking good for our wild Aussies. Without the repairing hand of a breeder to manually bob the tails of 1/3 of the offspring, the stubborn and arrogant Aussies would ostracize them as Border Collies, an extermination that would have to be carried out every generation, for the tail will always haunt the Aussie breed. Another 2/9 of the population is naturally tailless but in the decidedly mundane black coloring. Are they even worth altering? From looking at the entries at popular dog shows, the merle dogs easily outnumber the solid colors 2:1, meaning that at least in the show world Merle is about as saturated as it can get. There’s no way to tell from a distance if a tail is a natural bob or a surgical one, so the saturation of that allele is a question for science.
None of the tailed Aussies I’ve encountered were Merle and I’ve met a few. This anecdata suggests to me that breeders are less inclined to manually dock solid colored puppies to the standard (especially considering that one would expect to see twice the number of tailed-merle dogs vs. tailed-solid dogs if breeder’s were not docking), or perhaps Merle is more of a fetish in “conforming” show breeders than breeders who don’t worship the standard.
For the genetically inclined, here’s the same chart as above with the actual genotypes marked over the phenotype pictures. For the uninitiated, the four columns across the top can be thought of as the four possible sperm that the sire dog can produce and the four rows are the four possible eggs the dam can produce. Where they intersect represents the meeting of that particular sperm and egg. Sperm and eggs only carry one copy of genes so that when they combine there are two copies in the zygote. When we are looking at two different genes, there are four possible pairings. Four kinds of sperm meeting four kinds of eggs result in 16 possible outcomes, several of which are genotypically and/or phenotypically the same.
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