Predicting the properties of first cycle inbreds and second cycle hybrids extractable from two, three and four parent...

Standard biometrical genetical models of Mather and Jinks (1982) when made applicable to the means and variances of the 55 early generations produced by crossing four parents and six F1's in all possible combinations provide estimates of genetic parameters that can be used to predict the distributive properties of the first cycle inbreds and second cycle hybrids which could be extracted from any of these generations. Thus we can predict the inbreeding and outbreeding potentials of each generation in the early stages of a breeding programme and formulate the best breeding strategy for harnessing the full genetic potential of the breeding material and choosing the best end product. The 55 generations provide reliable estimates of the predictors and therefore should be used whenever possible. Simpler experiments consisting of the basic generations of the six single crosses, however, are sufficient for obtaining estimates of the predictors of the inbreds and can be used to predict their properties when information about the second cycle hybrids is not required or the remaining generations are not available. Replacement of the F2, backcross, three way cross and double cross generations with their randomly mated progenies is expected to improve the accuracy of predictions in the presence of a linkage disequilibrium. However, the gains made may not be justified against the costs of the additional breeding and the delay in making the predictions. Extensive experimental testing of the theory of these procedures must await the completion of the current field experiments. In the meantime the limited tests conducted on the material extracted from the Nicotiana rustica varieties V1, V2, V5 and V12 have confirmed the familiar conclusions that the V1 X V5 and V2 X V12 differ in their inbreeding and outcrossing potentials.