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A crop variety that has been produced by crossing two selected parental pure lines. F1 hybrids are favoured because they can combine the qualities of the parental lines and because they usually show hybrid vigour. However they do not breed true and seed must consequently be produced each year. In such seed production selfing within the parental lines, which would give nonhybrid seed, must be prevented. This may be achieved by emasculation and hand pollination but this is only commercially feasible for particularly valuable crops. Selfing can be prevented by ensuring that the parental lines both contain (different) reliable incompatibility alleles, i.e. S alleles high in the dominance series. This method is employed in producing F1 hybrid varieties of certain brassica vegetables. Alternatively one parental line may be male sterile; however use of male sterility halves the yield of hybrid seed since seed set is only possible on one parent line. F1 hybrid varieties of maize may be produced this way.
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