We measured the Norberg angle in 286 PWDs and calculated the heritability of this trait () finding a value close to that reported for coxofemoral joint laxity in German Shepherd dogs in an earlier study [Leighton et al., 1977
]. Significant heritability also was obtained from data for only the right or only the left joints, although the values were somewhat lower. The Norberg angle was independent of age and gender. There was no significant correlation between inbreeding coefficient and the Norberg angle. Thus, none of the variation in the Norberg angle could be attributed to inbreeding depression. Finally, as in previous studies of other breeds, the average Norberg angle was inversely correlated with radiographic evidence of osteoarthritis. Because most of the PW dogs in our study were young, the correlation was relatively weak (−0.24, P
= 0.00004). Radiological examination of these dogs as they age may lead to a stronger correlation.
A striking characteristic of our measurements is consistently greater laxity in the left hip (). This directional asymmetry is highly significant (Kolmogorov–Smirnov non parametric test: P < 10−12) and the polarity (left Norberg angle smaller than right) is observed in 80% of the dogs. However, this difference between the Norberg angles of the right and left joints did not show significant heritablilty. Thus, the asymmetry is probably the result of interaction with the environment. If specific genes are involved, they already are either fixed in the PWD population, or segregation is restricted to very few individuals.
In view of this polarity, we searched for QTLs using the right and left hips as separate data sets. Two significant QTLs were identified after correcting for pedigree effects (). Both are located on chromosome CFA 1 (), but are separated by about 94.6 Mb on the canine radiation hybrid genetic map [Guyon et al., 2003
].One of these, linked to FH2524, affected variation in the left hip and accounted for 14% of the heritable variation in that hip. It had no significant effect on the right hip. The other, linked to FH2598, was responsible for 16% of the heritable variation in the right hip but had no significant effect on the left hip.
Fig. 3 An ideogram of canine chromosome 1 (CFA01) highlighting locations of FH2524 and FH2598. Mapping data is based on a radiation hybrid map of CFA01 (Guyon et al., 2003, submitted) using the Multimap [Matise et al., 1994] and TSP Concorde Software programs (more ...)
This genetic asymmetry can be seen in the raw data and after correction for pedigree effects. presents the phenotypic means of the different marker genotypes associated with each QTL. The data for FH2524 have been sorted according to increasing values for the left hip after pedigree correction. The data for FH2598 were sorted in the same manner using values for the right hip. The maximum pedigree correction was about 2.0° (left hip) and 1.9° (right hip) for FH2524 and FH2598 genotypes respectively (the average corrections were 0.7° and 0.95°, respectively). Most of the corrections tended to bring phenotypic values closer to the mean, increasing values for the left hip of FH2524 genotypes and decreasing values of the right hip for FH2598 genotypes.
The frequency of homozygous genotypes for both FH2524 and FH2598 was consistent with the frequencies predicted from allele frequencies on the basis of random mating. Ten genotypes of the marker FH2524 and 13 genotypes of FH2598 were represented by fewer than five individuals. Among these were homozygotes of FH2524 (two dogs each of genotypes BB & CC and one of II); and homozygotes of FH2598 (one dog each of genotypes BB, CC, & GG).
These differences are seen graphically in . Cumulative distributions of phenotypic values associated with some of the genotypes are shown. For both markers, the non-heritable difference between the left and right hips (, upper versus lower panels) is apparent. However, the extent of phenotypic variation between genotypes is different for the two QTLs with greater variation in the left hip associated with marker FH2524, and more in the right hip associated with FH2598. We corrected for pedigree effects assuming that an additive mode of inheritance was responsible for most of the heritability of this phenotype. The range of corrected phenotypic means () associated with FH2524 genotypes is greater in the left hip (100°–111°) than in the right (107°–113°) and the opposite differential is observed for the phenotypes associated with FH2598 genotypes (left, 105°–109° <right, 106°–116°). These differences in variation are responsible for the differences in QTL significance (P-values, ) between the right and left hips.
Fig. 4 Distributions of Norberg angle metrics associated with particular genotypes of the markers FH2524 (a and b) and FH2598 (c and d). The left (top) and right (bottom) hip joints are shown. See for additional information as well as additional genotypes. (more ...)
The order of phenotypic values for the right and left hips corresponds for most of the genotypes. However, the FH2524 genotypes CF, CE, and EF are more clearly separated from DD, EE, and CD in the left hip; whereas FF, which was separate from CE and EF in the right hip is less distinct in the left (). The relative order of FH2598 genotypes EE, EH, and EG remain the same, but the indeterminate order of CH, CE, and EH in the left hip is resolved in the right hip where CH, CE, (and EG) have much larger angles ().
We have estimated the phenotypic means using an additive model of inheritance (). The QTL associated with FH2598 () fits this model well. However, there are few constraints on this fit, since we do not have adequate data to define the means of AA, CC, DD, FF, or GG. In , the genotype labeled GN represents a group of 13 dogs belonging to 6 genotypes containing the Gallele (1 AG, 1 CG, 1 FG, 1 GG, 4 DG, and 5 GH). It can be seen (, ) that the mean value for the EG phenotype is significantly larger than the means of the GN and EE phenotypes. This suggests that the phenotype of the EG genotype may result from an interaction between specific QTL alleles associated with the “E” and “G” haplotypes.
Fig. 5 Scatter graph of the pedigree corrected estimates of genotypic means (X-axis) graphed against the best fit to an additive model (a and b) for FH2598 and FH2524 (see “Materials and Methods”); or (c) graphed against an additive model with (more ...)
The QTL associated with FH2524 () does not fit an additive model. A simple dominant model (), in which one allele is assumed to be dominant to all other alleles, provides a slightly better fit but is not adequate. More complicated models involving specific interactions between alleles might improve the fit of FH2524 genotypes such as CD, CE, and CF as well as AD, AE, and AF or EF, BF and DF. Indeed, the data in suggest that for the QTL associated with this marker, C could be dominant to E but recessive to D; or that F may be dominant to C and E, but not to D or B.