HERITABILITY ESTIMATES OF HIP SCORES IN THE GOLDEN RETRIEVER BREED

Presented at the 27th Veterinary Orthopedic Society Meeting, Val Disere, France, 2000

Smith GK, Lafond E, Gschwend J, Fordyce H, Biery DN, Leighton EA, and Gregor TP: University of Pennsylvania School of Veterinary Medicine, Department of Clinical Studies, 3900 Delancey Street, PA 19104-6010

Introduction:

Estimates of heritability quantify the amount of genetic variation present for a given trait in a breeding population by expressing the proportion of total phenotypic variation that arises in this trait from genetic differences among dogs.  Heretability is a ratio of a part to its whole and therefore is represented by a number between 0 and 1.  An understanding of the heritability of a trait (eg. hip phenotype) is critical to determining whether genetic progress can be made by applying selection pressure for  (or away from) that phenotype.  To date, estimates of heretability of popular hip scoring methods used in this country have not been published.  Its was the purpose of this investigation to estimate the heribabilities of 3 hip scoring methods in a non-random population of Golden Retrievers.

Materials and Methods:

Breeders of Golden Retrievers were invited to volunteer litters and respective parents for radiographic evaluation of hip integrity.  Each dog was positioned for three radiographic exposures:  the hip-extended view, the compression view and the distraction view.  Only the results of the hip-extended and distraction views are included in this report.  The hip-extended radiograph was scored subjectively by a board-certified radiologist using OFA criteria.  In addition, hip laxity on this film was quantitated by means of the Norberg Angle (NA) measurement. Hip laxity on the distraction radiograph was scored using the distraction index (DI).

Estimates of heretability for each measurement were determined by invoking a simple linear regression of litter-mean hip score on parent-mean hip score.

The slope of the regression line is an estimate of heritability.  significance was set at P<0.05.

The study was 183 (91 males and 92 females, mean age = 11.2 months).  Of the 84 parents, 7 females produced two litters each, 9 males sired 2 litters each, and 3 males sired 3 litters each.  The mean age of the parents was 5 years.  The estimate of heritability of OFA-type subjective hip scoring was 0.221 but was not statistically different from 0.  In contrast, the estimates of heritability for NA and DI were 0.46 and 0.64, respectively, and both were statistically significant.  There was much data scatter in the plots of the regression lines making the 95% confidence interval of these estimates broad.

Discussion/Conclusions:

The higher estimate of heritability of the distraction index indicates (in Golden Retrievers) that more rapid genetic change can be expected to occur in response to the application of selection pressure.  Specifically by using the DI, genetic change will occur 40% faster than that expected with the NA scoring and 290% faster than that expected with OFA-type scoring.  A point of interest is that the square root of the estimate of heritability is the 'accuracy' of score.  One can appreciate that the square root of the OFA-type heritability estimate is 0.47 meaning that the chance of making the correct selection of a breeding dog based on that dog's individual hip score (i.e. without the benefit of pedigree data) is approximately equal to flipping a coin.  In contrast, the square root of the DI heritability estimate is 0.8, making for more accurate selection decisions.  Of course, even greater selection accuracy can be achieved by incorporating DI with pedigree information into a calculation of breeding value.

Acknowledgements:

This research was funded by The Morris Animal Foundation through grants from the Seeing Eye Inc. and the Golden Retriever Club of America.

Please Note: HD is a multi-allele gene and therefore can not be isolated.  With current evaluation methods we can not expect to "eradicate" HD in dogs of any breed.

Comments made by my Vet after reading this study and doing a synopsis for me:   

"What it is doing is comparing OFA, a measurement hip laxity based on the standard OFA view-NA, and PennHip (displacement index-DI).  The conclusion is that OFA is worthless-no improvement based on selection using OFA "grading".  Using NA there can be significant improvement.  Using DI there is the greatest rate of improvement-40% faster than using NA AND 260% faster than using OFA.  The DI ideally would be used with pedegree history for best results."