Phenotypic variance for each of several bristle number characters (abdominal, sternopleural, second and third coxal) was partitioned using both hierarchal and dialled designs. Heritabilities and genetic correlations were estimated from parent-offspring regressions and correlations and half-sib correlations.A high proportion of the genetic variance for abdominal bristle number was due to epistatic and sex-linked gene action, but most of the genetic variance for the other characters was additive autosomal.The genetic correlations among sternopleural, and second and third coxal bristle numbers were all high, but that between abdominals and sternopleurals was low, while those between abdominals and either second or third coxals were virtually zero. An appreciable proportion of the covariance between abdominal and sternopleural bristle numbers was non-additive genetic.The diallel method gave more reliable estimates of genetic parameters when non-additive or sex-linked genetic variation was present.

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A breeding programme to develop a line of chickens with a high incidence of tibial dyschondroplasia (TD) from Australian broiler stock is described. Despite the absence of a control flock, this programme has demonstrated that selection in a broiler population can rapidly increase the incidence of TD. There was consistent circumstantial evidence of the presence of a major sex-linked gene, the recessive allele of which is associated with an increased incidence of TD. The realised heritability estimates (obtained in the absence of a control flock) consistently exceeded 1.00, suggesting the presence of an environmental trend favourable to the onset of TD over the four generations of this investigation. There was a high maternal component or dominance genetic component, or both, for the inheritance of TD in the final generation, suggesting that environmental factors associated with the female parent line may influence the incidence of TD in broilers.

Although there is little experimental information on the effect of simultaneous selection for two quantitative characters on the magnitude of the genetic correlation between them, it is apparently generally expected that such selection for the two characters in the same direction will cause a negative change in the genetic correlation, and selection in opposite directions a positive change. Selection using independent culling levels was done for each of the four combinations of high or low third coxal bristle number with high or low sternopleural bristle number in Drosophila melanogaster for 22 generations. To estimate changes in the genetic correlation, realized genetic parameters were estimated from single-trait lines started from the base population, and from the two-trait lines after 10 and 22 generations of selection. Changes in the genetic correlation in individual two-trait selection lines were variable and unpredictable. At generation 22 concurrent two-trait selection had resulted in significantly larger realized genetic correlations than divergent two-trait selection, so that results were contrary to the generally accepted expectation.

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