The parents of a screening population must be cross-pollinated in all combinations in order to produce a sufficiently heterogeneous mixture for screening. There are two approaches. With open-pollinated crops, or when a male gametocide (see 7.11) is used, it is usual to rely on random cross-pollination, often called a 'random polycross'. With some crops, which have only a low rate of cross-pollination, such as beans, a marker gene can be used (see 7.12). Random cross-pollination has the advantage that it saves an enormous amount of work. Its chief disadvantage is that it lacks precision, and geneticists who are doing genetic research, in addition to breeding new cultivars, dislike it for this reason.

However, this is not a problem for the members of a plant breeding club, who are interested only in producing new cultivars.

Alternatively, when hand-crossing is used, precision is possible because the parents of each cross can be recorded, and the seed of each cross can be labelled and sown separately. But this makes for a very heavy workload. If the available person-hours of work are limited, it is usually preferable to devote that work to a large screening population, than to a detailed recording of a small population.

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