Implications of Genetic Variances in a Hybrid Breeding Program1

TLDR: The covariances for hybrids are given and then applied to considerations of the type of cross within which to select, amount of inbreeding needed in the parent lines and the importance of the order in which the lines are combined.

Abstract: FOR many years the use of commercial hybrids has been the standard procedure in corn. More recently, in other crops and in poultry, hybrids have become increasingly important. The breeder needs an objective method of deciding whether or not to produce hybrids. Even after having decided to embark on a hybrid breeding program, he is faced with many practical problems such as the amount of inbreeding needed in the inbred lines; choice of type of cross, single, three-way, or double cross, within which to select; attention that should be given to the order in which the lines are combined; estimation and interpretation of variances among crosses; and prediction of the performance of one type of cross from that of another. A framework is needed for hybrids which will lead to objective decisions. It must properly take account, and consequently must be informative, of the types of gene action. Such a framework is believed to have been developed in a study to be presented here. This study considers the covariances, in terms of genetic variances, between all possible pairs of relatives among single crosses, three-way crosses, and double crosses. Then in terms of the genetic variances and environmental variances, all of which are estimable, one can formulate comparisons. The covariances for hybrids are given and then applied to considerations of the type of cross within which to select, amount of inbreeding needed in the parent lines and the importance of the order in which the lines are combined. Applications to the problems of predicting the performance of one type of cross from that of another and of estimation are to be presented in later papers.