Are hybrid varieties justified?

In a cross-fertilized species, like maize, where there is an hybrid vigour, this paper discuss the justification of hybrids in comparison to lines and populations according to genetic effects. It is underlined that, even when at one locus there is no overdominance, i.e., no superiority of the hetero zygous as compared to the best homozygous, hybrids lead to the greater gene tic advance at short term. It is only at long term that the difference betw een hybrids, populations and lines can be reduced. Such an advantage is due to the fact that hybrids allow the immediate accumulation in one genotype of the favourable dominant genes dispersed in two complementary parents. Fu rthermore, overdominance or pseudo-overdominance, i.e., the simulation of o verdominance due to tight linkage of genes in repulsion, cannot be excluded . Economical aspects are briefly discussed.