Alleles of the growth hormone (GH) gene and GH receptor (GHR) gene were analyzed for association with juvenile body weight (HBWT), age at first egg (AFE), the hen-day rate of egg production (HDR), egg specific gravity (SPG), and egg weight (EWT) in a strain of White Leghorns. The particular strain segregated at near equal frequencies for two GH alleles defined by differences at three restriction fragment length polymorphisms (RFLP) and for two GHR alleles defined by a single RFLP. The GH genotype was significantly associated with AFE (P < or = 0.04) as well as HDR from 274 to 385 d (P < or = 0.04) and 386 to 497 d (P < or = 0.0003). The GHR genotype (haploid in female chickens) had trends for association with HBW (P < or = 0.06) and HDR from AFE to 273 d (P < or = 0.07). The effects on the egg quality traits SPG and EWT were not significant. Regression analysis revealed that HDR was associated negatively with AFE and positively with HBWT. The slope of the regression line of HDR on AFE varied with the GH genotype, with the effect that the differences in HDR between GH genotypes was relatively small in chickens with early AFE and large in chickens with late AFE. Similarly, the slope of the regression of HDR on HBWT varied between GHR genotypes, with the result that the effect of the GHR genotype on HDR in chickens with low HBWT was opposite to its effect in chickens with high HBWT. The complex relationship between genotypes and traits may reflect gene interaction and indicates that simple models based on additive gene effects may not be adequate for the dissection of the genetic architecture of quantitative traits.

A genomic clone spanning 16 kb of the GH receptor gene was mapped and used as a probe for identifying restriction fragment length polymorphisms (RFLPs) in chickens. Several strains of meat-type and egg laying chickens were found to segregate for an HindIII RFLP located in the intron preceding exon 4. The polymorphic HindIII site overlapped with a poly(A) signal. Association of the HindIII RFLP with traits was analyzed in a random-bred White Leghorn strain in three generations using either selective or random genotyping. Both methods revealed significant association of the HindIII+ allele (presence of the poly(A) signal) with an increased juvenile body weight (130 days of age). In two meat-type strains divergently selected for size of the abdominal fat pad, the HindIII+ allele was coselected with leanness. The results indicate the presence of a genetic variant of the GH receptor gene which affects growth and abdominal fat deposition and which is relatively frequent in egg laying as well as in meat-type chickens.