EVIDENCE ON THE GENETIC-CONTROL OF LH-RELEASE IN RESPONSE TO GNRH FROM CROSSES BETWEEN SELECTED LINES OF SHEEP

The purpose of this study was to determine the genetic basis of respon se to divergent selection for the amount of luteinizing hormone (LH) r eleased in ten-week-old lambs after an injection of gonadotrophin rele asing hormone (GnRH). Data from 518 animals in the selection lines, th e F-1 cross between the lines and both backcrosses were analysed by re sidual maximum likelihood to estimate crossbreeding genetic effects. T here was no evidence for sex-linked effects, maternal effects, sex-lim ited effects or dominance of the autosomal direct effects. Thus select ion responses have been achieved by changes at additive autosomal loci which have the same effect in both sexes. Maximum likelihood segregat ion analysis was used to look for a major gene affecting LH release. A n apparent effect of a major gene with a similar allele frequency in b oth selection lines was detected, but this result was attributed to re sidual non-normality which had not been removed by the transform appli ed. There was no evidence that a single major gene explained the diffe rence between the selection lines, but the study was not sufficiently powerful to rule out contributions from one or more genes of smaller, but still appreciable, effect.