Age-regulated expression of genetic and environmental variation in fitnesstraits. 2. Genetic effects and variances for viability among crosses from a factorial mating of six selected Leghorn strains

White Leghorn strains were crossed reciprocally in a complete factorial mat ing producing 6 pure strains and 30 strain-crosses, which were kept for two laying cycles: 133-496 d of age and 547-909 d. Hens were housed for lay in dividually in four two-tiered batteries of cages. Strain additive effects ( A(i)), strain sex-linked effects (Z(i)), strain-cross heterotic effects (h( ij)) and residual effects were calculated using regression. Viability was h igh in the first cycle of egg production with only 1 to 3% mortality in eac h of the four 11-wk periods, but lower in the second cycle decreasing with age. There was significant variation among strains in additive autosomal an d sex-linked genetic effects and strain-cross heterotic effects, which incr eased with age in the second cycle. Heterosis for viability was positive in some strain-crosses and negative in others with considerable changes with age. The magnitude of heterotic effects was generally greater than the magn itude of additive or sex-linked genetic effects for viability. These result s imply that different genotypes mount subtly different genetic responses t o the problems of viability with advancing age and that more than one theor y of ageing could apply. The results are discussed in relation to the theor etical aspects of ageing genetics.