High-resolution gametic map of the sheep callipyge region: linkage heterogeneity among rams detected by sperm typing.

The callipyge locus (CLPG) causing muscular hypertrophy in domestic sheep has previously been mapped to the distal part of ovine chromosome 18. In this study, an accurate multipoint linkage map consisting of six microsatellite markers in this chromosomal region was constructed based on the analysis of 1145 single sperm cells. The best supported order of markers was OARHH47-ILSTS54-MCM38-CSSM18-IDVGA30-BM S1561. The log odds against the second most likely order, which has a reversal of the closely linked markers CSSM18 and IDVGA30, was 5.026. Sperm typing can be used to examine a large number of meioses in single individuals, and therefore, was exploited to study individual variability of recombination rate in rams of different callipyge genotypes. The results revealed statistically significant linkage heterogeneity among rams (P < 0.05) for marker interval OARHH47-CSSM18, with individual recombination fractions varying from 0.209 to 0.357.