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Why are EBVs different for littermates?

The EBVs change due to their genetic merit.  When all puppies in the litter have no data, the genetic merit is estimated from only the parents.  As data accumulate, more of the differences become apparent.  Those differences are observable either by running a test (such as an x-ray or a noise test) or putting a harness on the dog and walking it around.  Those differences observable between dogs is the variance

Variances are calculated for the population of dogs in the dataset and reported in categories based on what is influencing the variances.  The EBVs are impacted by the genetically controlled portion of the variance and is reported as the genetic variance.  The rest of what is observed is due to environmental influences, changes over time in how the measure is made or scored and/or the unknown ‘residual’ portion.  These are labelled the environmental variancecontemporary group variance and the residual variance.  Each of these 4 variances account for the total differences between dogs in the dataset.  

The greater the portion of the differences that are attributable to genetic differences, the higher the estimate of heritability.  In fact the estimate of heritability is the genetic variance divided by the sum of all variances.

Accuracy of the measure (score, method to score, age old enough to see the trait) and the quantity of data on the dog and close relatives is very important.  Less data = lower accuracy.   Inaccurate measures increase the contempory group and/or residual variances.  When these variances are bigger, the portion that is attributable to genetic differences is small which means the estimate of heritability is lower for that trait.  The lower the estimate of heritability, the slower the response to selection.  

The PennHIP score, Harness Sensitivity testing, Thunderstorm Fear via the survey Guiding Eyes used and that they used it on all/most dogs through their lifetime means these traits have higher estimates of heritability because they are typically measured and scored consistently.   All 3 of these traits respond quickly to selection.

Additional measures of behavior by scoring BCLs at various age points using effective and consistent ways to measure the dogs will also increase the heritability of many of the behavior EBVs.  We know that because the IWDR estimates of heritability for Labradors is much lower than what Guiding Eyes has estimated.   It would be very helpful if individuals within organizations and between organizations are using consistent measures and scoring accurately – this is one of the goals of our BCL certification testing through the IAABC.  

12 months is not the ideal age for hip and elbow radiographs.  It would be best if these were done a little older if possible. 

The permanent environmental effect is changes in behavior influenced by the environment which stay with the dog but are not passed on to the offspring.  Some examples are the skill level of the puppy raiser while training and reinforcing the training and level and quality of socialization opportunities especially during the most critical period of birth through 16 weeks.

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