Six maize open pollinated varieties, three dents and three flints, their 15 F 1s, 15 F 2s and the 30 backcrosses together with six commercial double crosses as checks were evaluated for grain yield, in two years. The main findings were: an average heterosis of 18.8% was obtained by the F 1s relative to mid parent, with a range from 5.6% to 36.8%. ln, relation to the checks, the F 1s yielded on the average only 5.5% less, the best FI cross being 15.8% more productive than the checks. The yielding ability of the parents appears to play a significant role in the vigor of the corresponding crosses. There was very little decrease in yield from the F1s to the F 2s, of the order of 3.3%. Generation F 2 and BC were very similar and significantly superior to the parents. This is an indication that a great portion of the vigor observed in the F1 remains in the advanced generations, and thus must be essentially due to fixable genes, probably recombination and additive gene effects. Regarding endosperm types, only the crosses dents x dents were significantly superior to the corresponding flints x flints. All other comparisons involving endosperm types were nonsignificant, indicating that the character dent and flint per se is not responsible for a ·great portion of the heterosis observed. The two populations of exotic origin, Piramex (Tuxpan germplasm) and Piracar, a representative of Cuban flint germplasm, arc the best parents since they gave the best F 1. Both these parents appear to contribute dominant genes for high yield in crosses. The poorest parent in all respects was Cristal. Apparently it is recessive for genes of low yield and might be a good discriminating tester. Cristal ranked the other five varieties in the same order as did the overall F 1s. In general, deviations of F 2s in relation to parents, F 1s, and backcrosses are small. However, for one cross (Dente Riograndense x Piramex) and possibly for five others, these deviations arc of sufficient magnitude to suggest the presence of epistatic gene effects for yield.

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