|Image is from Flickr.com under a Creative Commons license.|
According to the tags on this photo, the owner believes this twelve pound dog is a Chihuahua/terrier/Australian cattle dog mix. He has a curled tail, and his ears are semi-prick. If anything, I can almost guarantee that this little fellow has no Australian cattle dog in him. First, I am going to do his genotype, then explain why I believe this statement to be true.
To begin with, this dog is tan point. The tan verges on creeping tan/capped tan since it nearly connects around his eyes and is so high on his legs. Either way, he must be genetically atat tan pointed. I highly doubt that he carries recessive black since it is both very rare in dogs, and also appears to not be present in any of his likely ancestors.
As for the intensity of red on this dog, it is in the moderate range. I suspect that he is genetically cchcch moderate red.
Next, this fellow is clearly merle, and thanks to his appearance and the fact that he is a mix, it is almost guaranteed that he is not a double merle. This would make him Mm merle.
Last, but not least, he has a fair amount of white on his body. His markings are in the usual range of Irish white, and as such he is most likely sisi Irish white.
So, that is atat cchcch Mm sisi or tan pointed merle with Irish white.
Now, back to his breed(s). I suspect that part of the reason why this dog is suspected of being part Australian cattle dog is thanks to his appearance. However, ACDs are never merle, but instead are extreme white piebald with heavy ticking. Yes, the two phenotypes can appear to be rather similar, but they are still rather simple to distinguish. For one, ACDs are born (mostly) white and develop color with age. Merle dogs are born they same color as they will be as an adult. Also, ticked dogs will not have prominent patches of white the way this dog does. Technically, ticked dogs will have white, but thanks to the ticking gene it will be obscured by colored hairs.
Anyway, if not from an Australian cattle dog, where did this dog get his coloring? Merle is generally only seen in herding breeds, however the color is actually quite known in the Chihuahua breed. This includes its acceptance by such registries as the AKC. In addition, large Chihuahuas, as well as those with curled tails and ears that do not fully stand up are not uncommon. Though it is possible that he may be a mix, I think that this dog may very well be a purebred Chihuahua.