breeders, while selecting for muscularity, have been careful to avoid the extreme double-muscled phenotypes. Thus, almost by accident, they have selected not only for the defective myostatin gene, but also for those other modifying genes, with the result that significant numbers of the breed (two-thirds) now carry a defective myostatin gene (with its associated advantages) but do not present the problems posed by those breeds with more extreme double muscling. Thus those breeders wanting the advantages of a defective myostatin gene without its disadvantages should consider using South Devon cattle.