Recently, a group of researchers were able to create transgenic beagles using CRISPR to selectively mutate the MSTN gene. This resulted in two puppies with different sets of MSTN mutations. Puppy #11 (dubbed Tiangou, after the Chinese mythical heaven dog) demonstrated similar muscle mass to littermate controls, due to being chimeric (i.e. some cells carried the mutation while others did not). However puppy #5 (dubbed Hercules after...Hercules) was shown to have biallelic MSTN mutations in all tissues resulting in the double-muscled phenotype observed in the aforementioned domesticated breeds.
Authors of this recent publication note that this could serve as an alternative model in the development of therapies for muscle wasting disorders, but also mention the potential to create "new strains of dogs with favorable traits for other purposes." The implication of designer pets brings to mind ethical concerns. Though essentially, all pure-bred dogs are freakish inbred wolves, mutated through the generations of artificial selection instead of direct genetic manipulation