Markers of alleles for three physiological candidate genes for reproductive traits, growth hormone (GHR), gonadotropinreleasing hormone receptor (GNRHR) and neuropeptide Y (NPY) were assessed for the association with the total egg production, number of double-yolked eggs and age at first egg in a single generation of a broiler breeder (Gallus gallus) pedigree dam line. Single-nucleotide polymorphisms and deletions were detected in the GHR, GNRHR and NPY genes. Genotypes were identified using a PCR-RFLP assay. The frequency of restriction enzyme þ /Àalleles in the population was for GHR 0.68 (NspIÀ) and 0.32 (NspI þ ), for NPY 0.78 (DraI þ ) and 0.22 (DraIÀ) and for GNRHR 0.54 (Bpu1102I þ ) and 0.46 (Bpu1102IÀ). Trait data from a total of 772 hens in 67 sire families from one generation of the pedigree dam line were recorded. However, the analysis used only the offspring of heterozygous sires to reduce the influence of selection and genetic background (n ¼ 33 sire families for GHR; n ¼ 14 sire families for NPY; n ¼ 36 sire families for GNRHR). A dominance effect of NPY on age at first egg and an additive effect of GNRHR on the number of double-yolked eggs were found (Po0.05).

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PHYSIOLOGY AND GROWTH PERFORMANCE S113 Reasons why hatchability was so low, and why embryonic mortality in the setter ( that is, pre-21 d) increased with the laying season, are as yet unclear.