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The gene encoding chicken growth hormone (GH) was isolated from a chicken genomic library. The size of the gene was 4 kb. It was digested with PstI and subcloned into pUC18. Three of the PstI fragments were used for restriction fragment length polymorphisms (RFLPs) analysis at the GH locus in two chicken strains (fat and lean line). Four polymorphic sites were detected using a PstI fragment (PII) as a probe. One polymorphism was located at a SacI restriction site (PS1), and three at MspI sites (PM1, PM2 and PM3). A method based on polymerase chain reaction (PCR) was developed for detecting polymorphisms at PM3 site. A fragment of 823 base pairs which contained the PM3 polymorphic site was amplified. Three genotypes (+/+,$-$/$-$ and +/$-$) were distinguished by examining the MspI digested PCR products in either agarose or polyacrylamide gel. / Ten anonymous cDNA clones were also isolated from a chicken liver cDNA library and used for RFLPs analysis. Three of these clones were found to be able to detected RFLPs at MspI sites in chicken strains (strain 7, 8, 9, 8R, S and K) indicating that a high frequency of genes are polymorphic and can be used as markers in mapping experiments. One of the three clones was present on a haploid genetic element. Segregation analysis showed that the inheritance of this haploid gene was determined by the genotype of the female parent.

Due to the importance of insulin-like growth factor-I (IGF-I) in regulating many physiological and metabolic processes, the IGF-I gene was chosen as a candidate gene to study trait associated polymorphisms in chickens. A PstI restriction fragment length polymorphism (RFLP) was detected at the 5' region of the gene and mapped to about 7 Kb upstream of the published promoter sequence. Analysis for association of the marker with traits of economic importance in an unselected, random-bred population of 359 White Leghorns revealed a significant association with egg weight (P ≤ 0.05) and specific gravity (P ≤ 0.05). There was also a trend for association with juvenile body weight (P = 0.08) but not adult body weight. For egg weight the PstI (-/-) genotype was associated with lower egg weight as compared to the heterozygote or the PstI (+/+) genotype. The PstI marker also was found to be significantly associated with differences in trait correlations. A regulatory loop that co-ordinated feed consumption, body weight, egg weight and rate of egg laying was detected, and this regulatory loop differed among the IGF-I genotypic classes. In the PstI (+/-) genotype, the degree of correlation between some of the traits was time dependent, while in the PstI (+/+) genotype it remained constant through the different periods of measurement. Since IGF-I is known to play an important role in immune functions, the association of the IGF-I genotypes with immune traits was also investigated. A significant association was found for delayed type hypersensitivity, interferon production and T-cell count (P ≤ 0.05). Individuals belonging to the PstI (+/-) genotypic class exhibited higher immune response, reflected by the delayed type hypersensitivity reaction and antibody the interactive effects of marker genotypes in the GH, GH-receptor and IGF-I genes on traits and trait correlations indicated that the three are part of an epistatic pathway, wherein the phenotypic consequences of

Chicken genomic DNA containing 5 kb of the 5$ sp prime$ end of the growth hormone (GH) receptor gene and 12 kb of the region up-stream was cloned and a restriction map was constructed. Using subcloned fragments as probes, a HindIII polymorphism was detected in both egg layer and in meat-type chickens. This polymorphic site was mapped at 7 kb up-stream of the coding region of the GH-receptor gene and a PCR assay for the polymorphism was developed to facilitate genotyping of large numbers of chickens. / Alleles of the GH-receptor gene and the GH gene were analyzed for association with traits in chicken strains of different genetic origins. In egg layers, association was significant for juvenile body weight, egg weight, feed consumption and feed efficiency for egg mass (P $<$ 0.05). In meat-type chickens, the GH-receptor allele associated with high juvenile body weight in egg layers was co-selected with leanness. A comparison of the genotype classes revealed that for several traits there was significant interaction between the GH and GH-receptor genotype. The results indicated that there are variants of the genes of the GH-axis which affect traits in White Leghorns and that the effect of a genetic variation in one gene may depend on the variation in another gene.

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