Often referred to as the “bully gene”. Myostatin is a growth factor responsible for controlling the development of muscle fibres. When the myostatin-producing gene is unable to function properly, excess muscling develops. Inheriting two copies of the mutation (one from each parent) results in a whippet with almost twice the amount of normal muscling. Although whippets with one copy of the mutation appear normal but may be more muscular than whippets not carrying the mutation, there is no real way to tell whether a whippet carries the gene just by its appearance. Double-muscled whippets exist in several unrelated lines in North America and Europe and, although this problem is now small, it appears to be growing.