Theoretical studies on plant breeding technique. IV. The estimation of genetic correlation between traits and its use in forest-tree breeding.

One of the main difiiculties in the breeding of forest trees is the long time taken by trees to mature, in some cases as long as 100 years. Consequently it is impossible to, determine either the heritability of any economic character or the gerLetic correlation coeffi:cient between any two economic characters of forest trees at maturity. This paper intends to explore the possibility of finding, theoretically, techniques ofi selection useful for groups of plants of such longevity. Figure I in the tex. t is a diagralrk showing relations among a character of standing mother-tree (P1) the same character of the offspring (P3), to be observed in future, and a differeLt but more or less similar character of the offspring (P2) in the nursery. Ei and Hi stand for the effect of i-th environment and that of i-th genotype on the i-th character Pi, respectively. rpaPJ is; the phenotypic correlation coefiicient between Pi and Pj while rHiHj is the genetic correlation coefficient between Hi and Hj. The letter hi in the figure is the square root of the. heritability value of the i-th character. For instance, we may assume that Pl be the, size of a mother tree, P2 the mean height of young trees grown in the nursery from seeds collected from the mother tree, and P3 the mean size of the offspring to be observed after 50 to 100 years. We can, then, hope to obtain the data for P1 and P2 only, and not that for P3 in our time. From Figure 1, we can get the following formulae, assuming open-pollination of the trees.