EVALUATION OF THE OVINE CALLIPYGE LOCUS - I - RELATIVE CHROMOSOMAL POSITION AND GENE-ACTION

Genotypic and phenotypic data were collected to estimate chromosomal p osition of the callipyge (CLPG) gene and to test gene action. Nine Dor set rams of extreme muscling phenotype and 114 Romanov ewes composed t he grandparent generation of a resource flock of 362 F-2 lambs segrega ting at the CLPG locus. The parent generation consisted of eight F-1 s ires and 138 F-1 dams. The F-2 lambs were serially slaughtered in six groups at 3-wk intervals starting at 23 wk of age to allow comparisons at different end points. A linkage group of 25 marker loci (mean of 7 08 informative meioses per marker) spanning 87.2 cM was developed and improved the previous known coverage and precision of marker order and interval distance from available maps of ovine chromosome 18. Probabi lities of each CLPG genotype were calculated at 1-cM intervals (0 to 1 07 cM). Statistical models included effects of year, sex, sire, regres sions on genotypic probabilities, and genotype-specific linear and qua dratic regressions on appropriate covariates. Orthogonal contrasts of CLPG genotypic effects evaluated additive, maternal dominance, and pat ernally derived polar overdominance models of gene action. The most pa rsimonious model did not include the additive and maternal dominance g enetic contrasts. From analyses of four key traits, a consensus for po sition of CLPG was obtained at 86 cM relative to the most centromeric marker. An F-test with 3 df representing polar overdominance was maxim um at position 86 cM (F = 407.4; P < .00001) with leg score as the dep endent variable. These results are consistent with assignment of the C LPG locus to the telomeric region of chromosome 18 and support the pol ar overdominance model of gene action proposed by Cockett et al. (1996 ). Furthermore, recombinant individuals with definitive phenotypes con fined the position of CLPG to a 3.9-cM interval, facilitating position al cloning experiments.