The main advantages of F1 hybrids are their vigour and high yield. These are achieved in the following way.

F1 hybrids derive from two closely-bred parental lines. These are initially selected based on specific desired traits. From these, a series of lines are inbred over several generations. This inbreeding reveals plants with recessive deleterious genes and these are rejected. The remaining healthy survivors are then crossed. With most deleterious genes eliminated, the progenies are very healthy and very vigorous . The plant breeder then selects the cross which has progeny with the most desirable set of traits – a new F1 hybrid.

Because each of the parents are highly inbred, each F1 hybrid progeny plant receives one near-identical set of chromosomes from each inbred parental line. As a result, each F1 seedling has a more-or-less identical set of paired chromosomes (although each set may be quite different – heterozygous for those of a technical bent) and so they are all very uniform as well as very vigorous.

The USA was the first country in which maize F1 hybrids became widespread and the shift from conventional open pollinated maize occurred very early, reaching almost 95% by the mid-1950s (Crow, 1998). Maize yields had been flat at about 1.5 tonnes/hectare until about 1940 but by the ’50s it had almost doubled to about 2.7t/ha. This increased yield was probably due largely to hybridity

But maize yields have continued to rise, now reaching 10.0t/ha. This extra yield can’t result from hybridity; hybrids were already prevalent! Instead, the early hybrids had lost some good genes in the process of getting rid of the bad ones and the later huge increase results from these good genes being returned to inbred parental lines plus extra beneficial genes gained from national and international collections (Duvick, 1999). Interestingly, these improved inbred parents are bred largely using ‘conventional’ (old-fashioned) plant breeding!

Perhaps surprising, genetically manipulated genes for herbicide and pest resistance have had little beneficial effect on maize yields despite being in almost all commercial American varieties; instead, they make it easier to grow the crop.