In the US they cannot be interbred though throughout the rest of the world, they can. Changes in social conditions may also fragment breed gene pools. The Australian Shepherd was originally a working ranch and farm dog. They are often registered as Australian Shepherds along with listing in a registry for minis. One of the results of gene pool fragmentation is loss of alleles that may exist in the breed but didn t happen to occur in the founders for that variety.
Genetic drift can cause further loss.
Many of these will be for things so subtle they might never come to a breeder s direct attention. A dog has some 100,000 genes, only a relative few of which are for things we can readily observe or measure. Many of these genes cause minor variations in form or bodily function. The use of popular sires, particularly multiple generations of them, can accelerate loss of alleles. A dog can only have a maximum of two alleles for any given gene.