Hemophilia is a potentially lethal blood-clotting disorder. It comes in two forms, termed A and B. Both are X-linked hereditary diseases, meaning the genes are on the X chromosome. Therefore females are healthy or only very mildly affected while males are severely affected. It is theoretically possible for a female to be affected if her sire was affected and her dam a carrier; if a female were to have the disease it would probably prove lethal at the time of her first estrous cycle.
The gene responsible for Hemophilia A, which governs coagulation Factor VIII, often mutates spontaneously so there will be no prior family history. One Hemophilia B gene has been identified as of this writing, but it is not the only one and it is possible that any given breed may have more than one mutation which can cause the disease. Hemophilia B arises from a deficiency of coagulation factor IX.
Both forms have been reported in Australian Shepherds but are extremely rare. Affected males should not be bred. Females who have one copy of the mutation will pass it to half of their female offspring, therefore females identified with mild hemophilia, dams of affected males and daughters of those dams should be removed from further breeding. Healthy sons of mildly affected or carrier females may be bred because they will have inherited the normal version of the gene. Breeding records of the dams and grand dams of identified carrier females should be scrutinized carefully to determine whether they were or were not themselves carriers; if male offspring succumbed to massive internal bleeding at a young age the females should be assumed to be carriers of hemophilia.