Age-regulated expression of genetic and environmental variation in fitnesstraits. 1. Genetic effects and variances for egg production in a factorialmating of six selected Leghorn strains

White Leghorn strains were crossed reciprocally in a complete factorial mat ing system producing 6 pure strains and 30 strain-crosses, which were kept in individual cages for two laying cycles, 133-496 and 547-909 d of age. Th e egg production in the second cycle (C2) of the various genotypes started about 10 - 20% lower and had a more linear and less persistent course than in the first cycle (C1). Strains exhibited very different patterns of age c hanges in both additive and non-additive genetic effects as well as in cyto plasmic effects. The additive autosomal and sex-linked genes (A(j) and Z(i) ) active in one laying cycle were quite different from those active in the other laying cycle as shown by low strain genetic correlations between thei r effects in C1 and C2. Further, the sets of A(j) and Z(i) genes responded with effects quite opposite to each other in both C1 and C2 as indicated by highly negative strain genetic correlations between the A(j) and Z(i) effe cts. The average non-additive genetic effect of sire strain i or dam strain j over ail its crosses with other strains (h(i)) and the non-additive gene tic effect due to the specific combination of genes occurring in each of th e two reciprocal crosses between strain i and strain j (s(ij)), showed very divergent patterns of age changes with a conspicuously greater divergence as age advanced. The overall non-additive genetic effect (mean heterosis) i ncreased significantly with age across the two cycles. The strain crosses t hat most successfully maintained their rate of lay until the end of C2, als o most successfully developed a rising age trend for total heterosis. The n on-additive genes active in one laying cycle were significantly different f rom those active in the other laying cycle as shown by the moderately low s train genetic correlations between their effects in C1 and C2. The genotypi c variance and its various components increased markedly with age, however, with a tendency to reach a plateau towards the end of both the first and t he second laying cycle. The environmental variance increased parallel to th e genotypic variance. Consequently, the phenotypic variance followed the sa me pattern of age changes. The results are discussed in relation to the the oretical aspects of ageing genetics. A model compatible with all the age tr ends of the genetic and environmental effects and variances is set up, assu ming that ageing is composed of two main opposing forces. Finally, the resu lts are briefly discussed from the animal breeding point of view.