Problems with Inbreeding in Pedigree Dogs

Introduction

     Inbreeding in pedigree dogs arises in because certain dogs, prized for exhibiting characteristics desirable for that breed, are used to sire many litters of puppies. The implications of this are that when the dogs from these litters come to be mated, some will inevitably be paired with dogs in other litters that have the same father. Over generations, more and more dogs across a particular pedigree are related to one another and the chances of relatives mating then increase. Genetic variability in domestic breeds of dogs, therefore, depends greatly on dog breeders’ decisions and practices(United Kingdom Kennel Club, 2011). Selection for particular types of animals can result in bottlenecks within the populations, leading to higher rates of inbreeding in dogs. Moreover, in dogs, the mating between close relatives is frequently used ( Ubbink et al., 1992) in the practice of line breeding. Mortality of puppies significantly increases with inbreeding (Van der Beek et al. 1999) and a positive correlation has also been shown between the frequency of some genetic diseases and the average coefficient of inbreeding (Ubbink et al., 1992). Moreover, purebred dogs often have to deal with genetic diseases and more than 400 genetic diseases are registered in dogs (Nielen et al. 2001). This paper looks at genetic diversity and the incidence of inbreeding in purebred dogs as well as the consequences of these breeding practices on the welfare of animals, in particular inherited genetic disorders.

Problems associated with inbreeding pedigree dogs

Loss of genetic diversity

     A study recently carried out by Calboli et al. (2008) investigated the incidence of inbreeding in purebred dogs. Through the development of novel approaches for examining population structure as well as using previously derived formulae  to represent inbreeding, Calboli et al. (2008) examined the electronic pedigrees of 10 breeds of dogs (a total of 2.1 million dogs) from the United Kingdom Kennel Club’s databases. Extremely inbred dogs were identified in every breed and the effective population size was estimated to be between 40 and 80 dogs for all except two breeds (Calboli et al. 2008). Distressingly, the effective population size of these breeds was several orders of magnitude smaller than the actual number of dogs registered in each breed. It was also found that in all but three breeds over 90% of unique genetic variants are lost over the course of six generations (Calboli et al. 2008). This points to the dramatic effect that breeding patterns can have on genetic diversity. While this was a large-scale study of many generations of pedigree dogs, it primarily focused on the more popular breeds.  The incidence of inbreeding and loss of genetic diversity could therefore be much higher in rarer breeds with smaller populations (Calboli et al. 2008). Such small effective population sizes mean that the chances of a dog breeding with a close relative, resulting in birth defects and genetically inherited health problems, are high.

Genetic Disorders

       Most diseases are affected to some extent by both genes and the environment. A genetic disorder is one in which a variation in the genetic make-up of the individual plays an important role in causing the condition (Canine Inherited Disorders Database, 2011). Although some disorders occur because of spontaneous mutation, many genetic disorders are inherited. These conditions are seen quite often in dogs, most of which are purebred dogs.
     The frequency of inherited conditions can be reduced through good breeding practices (Canine Inherited Disorders Database, 2011). However, for this to occur, we need to know how the disease is inherited, how to identify the condition in its earliest stages, as well as find ways to recognize carriers of the disease who are usually asymptomatic. Breeds that have a higher risk for a condition, in relation to other dog breeds, or to the general dog population are said to have a breed predisposition (United Kingdom Kennel Club, 2011) . Common sense suggests that these disorders are inherited, but for many breeds and for many disorders, the studies performed to determine how the disease is inherited or the frequency of the disease in a particular breed have not been carried out, or the results are inconclusive (Canine Inherited Disorders Database, 2011).
     One such disorder is canine hip dysplasia. Hip dysplasia is the most common inherited orthopaedic disease in large and giant breed dogs, and occurs frequently in many medium-sized breeds as well. Canine hip dysplasia is caused when the hip joint, (a "ball and socket" joint, in which the "ball”, the top part of the thigh bone or femur, fits into a "socket" formed by the pelvis) and the ligaments which help to hold them together are loose (Canine Inherited Disorders Database, 2011). This leads to the ball sliding part way out of the socket. Canine hip dysplasia is a condition that becomes progressively worse over time and your dog will become painful, lame and weak in the hind end (Canine Inherited Disorders Database).
     The mode of inheritance of canine hip dysplasia is polygenic. Polygenic traits are controlled by an unknown number of genes and in the case of canine hip dysplasia scientists do not yet know which genes or how many genes are involved (Canine Inherited Disorders Database, 2011). Because it is virtually impossible to determine the exact genotype for such traits, it is difficult to control defects with a polygenic mode of inheritance. The best attempts at control are based on a grading scheme for identification of the defect and a breed policy of recording and publishing the results for as many dogs as possible (Canine Inherited Disorders Database, 2011). Canine hip dysplasia remains a problem in most large breeds of dog, regardless of efforts to control this condition dating back to the 1960s(United Kingdom Kennel Club, 2011). Breed organizations and veterinarians in various countries have developed control programmes that rely on radiographic evaluation and a central registry of dogs (United Kingdom Kennel Club, 2011). Thoughtful selection by breeders, using this information, has greatly reduced the incidence of hip dysplasia in those breeds.
Conclusion

    In light of the many problems associated with inbreeding actions to either maintain or increase genetic diversity in the populations of purebred dogs should be a high priority for the health these dogs. Possible actions include limits on the use of popular sires, encouragement of matings across national and continental boundaries, and even the relaxation of breed rules to permit controlled outcrossing( McGreevy et al, 1999; McGreevy, 2008).

References

F.C. Calboli , J. Sampson, N. Fretwell, D.J. Balding. 2008. Population Structure and Inbreeding from Pedigree Analysis of Purebred Dogs. Genetics. 179: 593-601.

Canine Inherited Disorders Database. Canine Hip Dysplasia. Accessed September 28, 2011.

P.D. McGreevy, F.W. Nicholas . 1999. Some Practical Solutions to Welfare Problems in Dog Breeding. Animal Welfare. 8: 329-341.

P.D. McGreevy. 2008. Sick as a dog: Outraged by the health problems pedigree dogs suffer. New Scientist. 200:2677.

A.L. Nielen,S.  van der Beek, G.J. Ubbink, B.W. Knol. 2001. Population parameters to compare dog breeds: differences between five Dutch purebred populations. Vet. Q. 23: 43–49.

G.L. Ubbink, B.W. Knol, J. Bouw. 1992. The kinship between homozygosity and the occurrence of specific diseases in Bouvier Belge des Flandres dogs in the Netherlands. Vet. Q. 14: 137–140.

United Kingdom Kennel Club. Dog Health: Summary results of the Purebred Dog Health Survey for all breeds. Accessed September 29, 2011.

S. Van der Beek, A.L. Nielen, Y.H. Schukken, E.W. Brascamp. 1999. Evaluation of genetic, common-litter, and within-litter effects on preweaning mortality in a birth cohort of puppies. Am J. Vet. Res.60: 1106–1110.