GENERAL AND SPECIFIC COMBINING ABILITY FROM DISCONNECTED PARTIAL DIALLELS OF COASTAL DOUGLAS-FIR

General and specific combining ability (GCA and SCA, respectively) wer e examined in 36, 6-parent disconnected partial diallels across 4 diff erent experimental series in coastal Douglas-fir (Pseudotsuga menziesi i var. menziesii DOUGL.) to examine the ratios of the 2 genetic varian ces, the distribution of GCA and SCA effects, and estimates of genetic gain from GCA and SCA for 3 growth traits. Height at age-7 and height and volume at age-12 were measured on approximately 150 trees per ful l-sib family in each diallel, across 11 different test sites within ea ch series. The average percentage ratio of SCA variance to GCA varianc e was 36%, across all series and the 3 growth traits, with a range of 19% to 65%. GCA and SCA variances did not appreciably change for heigh t growth from age 7 to age 12. Diallel set effects were generally negl igible. From theoretical considerations assumed for the diallel model, clear separations of additive and dominance effects (vis-a-vis the as sumptions of selecting on GCA and SCA variances) are likely not possib le: the effects are subject to degrees of dominance, epistasis and Lin kage in the population. However, these genetic details did not manifes t themselves in any noticeable pattern or correlation among GCA and SC A effects. While these results confirm current strategies in Douglas-f ir breeding to select primarily on GCA, there are opportunities to uti lize SCA variance in the production population. Controlled matings for elite production populations are now common in coastal Douglas-fir im provement programs, and controlled crossing with specified parents cou ld i) elevate gains by as much as 3.0% in 12-year volume (assuming com petition effects have not biased volume estimates), and ii) access add itional sets of parents that would otherwise be disregarded. Gains for height growth from utilizing SCA are lower, which reflect either, i) lower SCA variances associated with height, or ii) competition has bia sed upward volume SCA variance by age 12 (relative to height).