Does inbreeding cause inherited disease?



Quote from the Kennel Club:

Does inbreeding cause inherited disease?

The gene mutations that result in inherited disease occur at random and are fairly rare events. We know from experimental data that certain chemicals and certain kinds of chemicals can cause DNA damage resulting in the mutation of the gene involved. Some of the inherited diseases that we recognise today will have resulted from such insults, but probably most result from errors in copying DNA that have gone undetected. So, inbreeding per se does not cause genetic mutations, as far as we know. However, we have already seen that many of the inherited diseases in the dog result from recessive mutations; an affected dog will have two copies of the recessive mutation. However, the carrier dog, that carrying one normal gene copy and one mutant gene copy, will be clinically normal. Inbreeding to such carrier dogs will result in a rapid build up in the frequency of the mutant gene such that eventually affected dogs will be produced. Inbreeding therefore does not cause a mutation that results in an inherited disease, but once such a mutation has occurred, inbreeding will increase the frequency of the mutant version of the gene in the breed far quicker than other more random breeding programmes.


Quote from the Kennel Club:

Hip Dysplasia

What is Hip Dysplasia?

Hip dysplasia is a condition where the hip joint does not develop correctly. As the dog gets older the joint undergoes wear and tear and the joint deteriorates, leading to a loss of function.   This can cause varying degrees of pain, discomfort, stiffness and lameness.

How is Hip Dysplasia inherited?

Hip dysplasia is a complex inherited disorder, which is influenced by a number of different genes and by several environmental factors (eg diet, exercise, environment or factors when in the womb before birth). Each of the genes that help to make a dog's hips may have different possible versions.  Some versions increase the risk of hip dysplasia, while others decrease the risk.  Each dog will have a mix of these "good" and "bad" versions of genes, making it very difficult to predict to what extent a dog will be affected.  The impact one version of a gene has might only be slight, but lots of genes having a small influence will have a combined additive effect.  The way in which these conditions are inherited is not straight forward; hence the name complex disorders.  These complex diseases are usually seen across many different breeds and are also described in both cross breeds and mixed breeds.

Which breeds are affected?

Since hip dysplasia is partly inherited, dogs that share similar genes are more likely to be similar in terms of how severely they have hip dysplasia.  Individuals in each breed share a significant amount of their genetic make-up and so certain breeds are more vulnerable to hip dysplasia than others.  It is generally accepted that this condition is more common in larger breeds (possibly because these breeds are heavier and this greater weight has more of an effect in producing osteoarthritis due to laxity) but can occur in any dog of any size, regardless of whether they are purebred or mixed bred.

The BVA/KC Hip Score Scheme

The British Veterinary Association (BVA) and Kennel Club (KC) scoring scheme has been in operation since 1984.  The scheme screens animals for faults in the hip joints and allows animals with the best hip joints to be chosen for breeding.  A dog's X-rays are scored by a panel of experts who are part of the hip dysplasia scheme.

Each hip joint is assessed by BVA/KC scrutineers who assign points based on nine aspects of each hip joint.  The degree to which a dog is affected by hip dysplasia is represented by a score given to each hip.  This score ranges from zero to 106 (zero to 53 for each hip) with a score of zero representing the least degree of hip dysplasia and 53 representing the most.

Using BVA/KC hips scores to produce Estimated Breeding Values (EBVs)

Data from the BVA/KC Hip Dysplasia Scheme is used to calculate an estimated inheritance "risk factor" for each dog (EBV).  EBV's help breeders of KC registered pedigree dogs to continue to make sensible and informed health choices for breeding, based on robust data.

Complex inherited disorders, such as hip dysplasia, are influenced by both genetic and environmental factors.  EBV's essentially strip away the environmental influences to estimate the genetic component of these conditions, enabling breeders to better understand the genes a dog may pass on to its offspring.  If used as part of breeding decisions EBV's can help reduce the risk of puppies inheriting hip dysplasia more effectively than by the sire and dam's individual hip scores alone.

EBVs link all available pedigree information with data collected through the BVA/KC testing schemes.  The information on relatives (who share genetics) influences the genetic risk of an individual.  Therefore, the more breeders that make use of these schemes, and the more dogs that are scored, the more accurate the EBV will be.  By continuing to hip score, breeders are securing the future for countless other dogs by providing the information needed to continue Estimated Breeding Values.

What do EBV values and figures mean?

The Kennel Club's EBV resource provides the hip score, the EBV as a number, the confidence as a percentage, and a graphical representation of the EBV and confidence.

EBV Scores

The breed average is always set to zero.  Dogs with a higher than average risk of passing on genes associated with hip dysplasia will have an EBV higher than zero.  Dogs with a lower than average risk of developing hip dysplasia will have an EBV lower than zero (ie a negative number eg -10).  The further a dog's EBV is from the average, the higher or lower its genetic risk.  A dog's EBV can change during it's lifetime, either upward or downward, as more data becomes available, either about the dog itself or its relatives.  At birth, a puppy's EBV will be the average of its parents' EBVs.  For example, a sire with an EBV of -5 and a dam with an EBV of +5 will produce a litter of puppies with an EBV of zero.


The confidence is an indication of how much scoring information has been used to calculate the EBV.  The more scoring information available from the dog itself and/or its relatives, the more confident we are that the EBV is close to the actual genetic risk.  The confidence of the EBV can increase if more relatives are, or the dog itself is scored.

* A dog with just its own hip score, and no relatives scored, will have a confidence of about 60%
* A dog without its own hip score but with the scores of both parents will have a confidence of around 40%
* A dog without its own hip score but with only one parent scored will have a confidence of around 30%


Where EBVs are available for your breed

EBVs provide a more accurate measurement of genetic risk than using a dog's hip score alone.  It is therefore recommended that breeding decisions are principally made using EBVs.

Ideally breeders should use dogs that have an EBV which is lower than average (ie a minus number) and preferably with a confidence rating of at last 60%.  EBVs with a confidence less than 60% can still be used, but the higher the confidence the more accurate the EBV will be.  A dog does not need to be hip scored in order to have an EBV but to ensure an EBV is as accurate as possible it is recommended that breeding dogs are scored under the BVA/KC scheme.  This is also important in understanding an individual dog's health risks for being at risk of developing hip dysplasia.

The lower the EBV the better, but breeders do not need to seek out dogs with the lowest risk EBV.  Selecting animals with a lower risk EBV than average will still lower the risk of hip dysplasia.

It is recommended that breeders make well balanced breeding decisions.   Each puppy will have an EBV that is the average of its parents.  Therefore dogs with an EBV which is higher than average can still be bred from, providing that it is mated to a dog with an EBV which is well below average (assuming that the confidence for both dogs is high) and the EBV of the resulting puppies is lower than the breed average.

Previously, the best advice was to use dogs with hip scores below the breed average, which meant that many dogs could have been excluded from a breeding plan if their scores were a significant consideration.  Excluding dogs from a breeding plan can have an impact on the genetic diversity.  By using EBVs it is reasonable to use a dog with an individual BVA/KC score over the breed average, as long as the EBV indicates low genetic risk with good confidence.  In such cases the hip condition of the offspring should be carefully monitored and preferably hip scored themselves.

Where EBVs are not available for your breed

An average (or median) score is calculated for all breeds scored under the scheme and advice for breeders is to choose breeding stock with hip scores around and ideally below the breed median score, depending on the level of hip dysplasia in the breed.  It is recommended that hip scores of a dog's family members should also be considered.

Making balanced breeding decisions

As well as considering the implications of a dog's EBV or hip score, there are other equally important factors to consider when deciding whether two dogs should be mated together, such as temperament, genetic diversity, conformation, other available health test results, the general health of the dogs, etc.  Your breeding decisions should always be well balanced and take into consideration the qualities and compatibility of both the sire and the dam that you are considering.

Can the results of the scoring scheme or EBVs be used to precisely predict if future puppies will be affected?

Hip dysplasia is a condition which is inherited in a complicated way not yet fully understood by scientists.  Due to the complex nature of inheritance of this condition it is still possible that affected offspring may arise from parents which have good EBVs.  It is hoped that breeding appropriately from screened dogs will reduce the risk of producing affected offspring and using EBVs reduces this risk even further, but it must be stressed that this is not a guarantee.

Will a DNA test for hip dysplasia be developed in the near future?

It is unlikely.  Hip dysplasia is a complex inherited disorder, caused by a number of different genes and also influenced by several environmental factors.  Traditional DNA tests can be developed for conditions controlled by only one gene to predict whether a dog will be clear, a carrier or affected, but not for conditions controlled by more than one gene.  For a disease such as hip dysplasia, even if the effects of all the genes involved were known, a DNA "test" would give a genetic risk of the disease.





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