Quantitative inheritance in wheat

The inheritance of yield, yield components and some morpho-physiological traits such as, plant height, spike length, and flag leaf length was studied in six New Zealand wheat cultivars. Three biometrical methods namely, the Half Diallel (Morley Jones, 1965), the Scaling Tests (Mather and Jinks, 1971) and the General Methods for Detecting Epistasis (Chahal and Jinks, 1978) were used for the genetical analyses of these traits. The genetical control of flour protein was also studied by The Half and Full Diallel analyses. Three principal components of yield, spikes per plant, grains per spike and 1000 grain weight were found to be under complex genetical control involving epistasis. However, grains per spike can be reduced into its basic subcomponents of grains at the individual spikelet position and spikelet, per spike to explore the role of multiplicative epistasis. In exact algebraic expression, Grains per Spike = ⁿ∑₁ ₌₁Gi where Gi is grain number at a specific spikelet position and N is the number of spikelets per spike. Using this rationale, an attempt was made to resolve the role of multiplicative epistasis in grains per spike by studying the nature of gene action of this trait and its two subcomponents. Grain number at a single spikelet position was shown to be under relatively simple additive gene control and spikelets per spike was under additive and dominance gene control. The simpler inheritance of these two subcomponents compared to grains per spike, which was shown to express epistasis, has enabled a selection strategy to be proposed. This selection procedure involves the direct exploitation of mainly additive genes for grains at a single spikelet position in early generations. A selection response can conceivably be achieved through this procedure instead of trying to fix the elusive large spike controlled by epistasis in early generation. The genetical analysis of morpho-physiological traits have revealed plant height and spike length to be under control of additive and dominance genes for all cultivars studied except for plant height in the cultivar Atlas 66. Epistasis for plant height was exhibited by the families derived from Atlas 66 and Karamu. The significance of this record of duplicate type epistasis was discussed in the light of the 'tall dwarf' breeding strategy. It was concluded that while the 'tall dwarf' selection model could be adopted for crosses between the semidwarf and the standard height cultivars studied, difficulty could be anticipated in crosses involving Atlas 66 and the semidwarf cultivar, Karamu, because of epistasis. The Diallel analyses of flour protein content in the F1 and F2 families indicated the genetical control for this trait to be mainly of the additive and dominance type. High narrow sense heritability was recorded, emphasizing considerable prospects for early generation response to selection in crosses between the high protein cultivars. However, epistasis was detected in the F1 Half Diallel analysis involving the cultivar, Karamu. The failure to record similar epistasis in the F2 Half Diallel analysis suggested existence of genotype environmental interaction for flour protein.