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We are grateful to Bet Hargreaves from Scotland who has written about :

In Breeding

The following has been summarized from a paper by Susan Thorpe-Vargas Ph.D., and John C. Cargill, M.A.,M.B.A.,M.S. which was published in the June 2001 DOGworld an American publication.

It is a genetic fact that the deleterious effects of in-breeding is what is happening to the immune system. More and more are now seen such problems as auto-immune diseases, irritable bowel syndrome and various food and environmental allergies. The genes that control the immune system must have the ability to differentiate foreign proteins from "self" without over reacting to these environmental perils.

The genes that control the immune system are passed down together, one set from each parent, they are found close together on the chromosome. When in breeding occurs the chance that a puppy will inherit an identical set of these genes from each parent increases and this cuts the functional ability of the immune system in half and seriously compromises the quality and duration of life for the puppy.
Those of you who have had a Cavalier with allergies with demodectic mange or without the ability to fight off a deadly disease, know the tremendous suffering this involves both for the dog and its owner. So there are environmental reasons for an impaired immune system but the bulk of literature suggests that in breeding plays the greatest role.
As is well known our Cavalier Breed was founded on a small number of stud dogs and to get our breed established Mother was mated to Son, Father to Daughter, Brother to Sister, thus a genetic defect that was very rare in the source population now can be very common in a particular breed, because one or more individuals in the new population carried that defect.

Since there are pedigrees of Cavaliers with in breed coefficients of 44.6% in the 1940's perhaps greater care should be taken by Cavalier breeders when planning their breeding programs and also a study of pedigrees so as to avoid in breeding and paying particular heed to have as many long living Cavaliers in the pedigree background as possible, since those Cavaliers have proved their health status by living to a normal old age.