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 response to divergent selection for the amount of luteinizing hormone (LH) released in ten-week-old lambs after an injection of gonadotrophin releasing hormone (GnRH). Data from 518 animals in the selection lines, the F1 cross between the lines and both backcrosses were analysed by residual maximum likelihood to estimate crossbreeding genetic effects. There was no evidence for sex-linked effects, maternal effects, sex-limited effects or dominance of the autosomal direct effects. Thus selection responses have been achieved by changes at additive autosomal loci which have the same effect in both sexes. Maximum likelihood segregation analysis was used to look for a major gene affecting LH release. An apparent effect of a major gene with a similar allele frequency in both selection lines was detected, but this result was attributed to residual non-normality which had not been removed by the transform applied. There was no evidence that a single major gene explained the difference 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.