TRAIT ASSOCIATION OF GENETIC-MARKERS IN THE GROWTH-HORMONE AND THE GROWTH-HORMONE RECEPTOR GENE IN A WHITE LEGHORN STRAIN

Alleles of the growth hormone (GH) gene and GH receptor (GHR) gene wer e analyzed for association with juvenile body weight (HBWT), age at fi rst 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 all eles defined by differences at three restriction fragment length polym orphisms (RFLP) and for two GHR alleles defined by a single RFLP. The GH genotype was significantly associated with AFE (P less than or equa l to 0.04) as well as HDR from 274 to 385 d (P less than or equal to 0 .04) and 386 to 497 d (P less than or equal to 0.0003). The GHR genoty pe (haploid in female chickens) had trends for association with HBW (P less than or equal to 0.06) and HDR from AFE to 273 d (P less than or equal to 0.07). The effects on the egg quality traits SPG and EWT wer e not significant. Regression analysis revealed that HDR was associate d negatively with AFE and positively with HBWT. The slope of the regre ssion 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. Simila rly, the slope of the regression of HDR on HBWT varied between GHR gen otypes, 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 hig h HBWT. The complex relationship between genotypes and traits may refl ect gene interaction and indicates that simple models based on additiv e gene effects may not be adequate for the dissection of the genetic a rchitecture of quantitative traits.