 Most foals appear normal at birth, with symptoms noticeable at an average age of four months, though there have been cases where the condition is first seen shortly after birth and other cases where symptoms are first recognized in horses over one year of age.Breeds DNA tested that reveal some carrier lines, but to date no affected animals, include the Welsh pony and the Trakehner.However, other breeds heavily influenced by Arabian breeding, such as the Thoroughbred and the American Saddlebred, do not appear to carry the mutation.