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Estimated Breeding Values (EBVs) for Hip Dysplasia

An Estimated Breeding Value (EBV) is an estimate of the genetic potential of an individual to pass on the genes associated with a specific attribute (either positive or negative) to its progeny.

How will EBVs help Bearded Collie breeders?

EBVs are particularly useful when an attribute is influenced by both the individual’s genetic make-up and by factors in the environment. In these situations it is often hard to distinguish between environmental effects and the effect of the underlying genes, making it difficult for breeders to select a sire with the right genetic potential to pass to future generations.

Hip dysplasia is a complex health condition influenced by both genetic and environmental factors. Breeders wanting to select against hip dysplasia currently rely on hip scores, which are a measure of observed hip attributes. However, these are poor predictors of the genetic potential (risk) of hip dysplasia inherited by a dog’s progeny. By focusing on the genetic predisposition or risk of the individual, EBVs provide breeders with a valuable resource.

The majority of Beardies have hip scores which fall into a relatively narrow range. In these circumstances, selecting dogs with hip scores around or just below the mean may not lead to any noticeable improvement over time because of the margin of error in the scores as predictors of inherited genetic risk.

EBVs help breeders by making it clear whether a sire’s hip score is associated with a genetic risk higher or lower than the breed average. Since only the genetic risk is inherited, knowing the EBV should help Bearded Collie breeders make faster progress towards reducing any predisposition to hip dysplasia in the breed.

How are EBVs calculated?

EBVs can only be produced if there is an existing widely-used way of measuring the observed attribute, and the extent to which inherited genes influence the condition (its ‘heritability’) is known. A commercial statistical package widely used in animal breeding and genetics is used to do the complex linear mixed-effect modelling calculations involved.

EBVs are calculated by jointly assessing all the dogs in the chosen population, using information about pedigree relationships, hip scores, heritability and environmental factors known to have an effect. The model takes account of the flow of genes from parents to their progeny, correcting for the effect of different elements on each other to estimate how much of the observed attribute in any individual is due to the genes he/she has inherited.

diagram showing model for EBV calculation

What do the results mean?

The EBV results are expressed as so-called ‘standard’ scores, which are often used to compare factors measured in different units which have a different average and degree of spread. They are scores which are adjusted so that they always have the same mean and standard deviation. Using standard scores makes it easier to compare an individual’s EBV with the average for the breed, and also to compare results for different conditions (e.g. hip scores and elbow scores). In the Kennel Club’s model, the EBVs for each breed are converted to standard scores from +120 to -120, with a breed average (mean) of 0, and the standard deviation of 20.

  • Positive EBVs - numbers higher than zero (the red area on the left of the centre bar) – show individuals with the genetic potential (risk) for hip dysplasia which is higher than the breed average.
  • Negative EBVs – numbers less than zero (the green area on the right of the centre bar) – show individuals with the genetic potential (risk) for hip dysplasia which is lower than the breed average.
  • The EBV is shown as a circle on the coloured bar. The shading around it indicates the accuracy of the EBV.
  • The further a dog's EBV is from the average, the higher or lower the genetic potential (risk) for hip dysplasia passed on to its progeny.

Since EBVs are estimates, it is important for breeders to know how accurate the estimate is. The confidence measure shows how accurate the EBV is compared with the individual’s true value. The higher the percentage, the more confident breeders can be that the dog’s genetic potential has been accurately assessed. The percentage is determined by the amount of information on the dog and its relatives used to calculate the EBV. As a rule of thumb:

  • anything below 40% should be regarded as a preliminary estimate of EBV which could change substantially with more information
  • levels above 80% have good accuracy.

It is easier to get an accurate EBV for sires than for dams, because sires typically have a greater number of progeny, who contribute scores.

How should EBVs be used?

The Kennel Club’s advice on using EBVs is:

  • the lower the EBV the better
  • however, selecting any dog with an EBV lower than average will still lower the risk
  • a dog with a hip score a bit over the breed average could be a reasonable choice as long as the EBV indicates low genetic risk with good confidence
  • selecting against hip dysplasia should be balanced with other important factors such as temperament, and health test results.

It is important to maintain genetic diversity while selectively reducing the risk of hip dysplasia. Ensuring that every mating makes some contribution to reducing genetic risk is an effective way of doing this as it avoids an undue focus on particular sires with low EBVs.

Trends

EBVs for hip dysplasia were launched at Crufts 2014, so as yet there is little information about their impact.


Last updated: 3 August 2015