Two kinds of zygosity

Open-pollination leads to heterozygosity, and self-pollination leads to homozygosity. Johannsen's (1903) discovery of pure lines showed the importance of this difference, and this gave further support to the Mendelian breeding method. A self-pollinated species can be made into a pure line by selfing for about six generations, and by selecting the best individual in each generation as the parent for the next generation. A pure line has a high degree of homozygosity, and it has the valuable agricultural character of 'breeding true'. Many of the most important food crops, such as wheat, rice, and beans, are self-pollinated, and they are now cultivated as pure lines. Open-pollinated species are usually cultivated as clones or hybrid varieties in order to preserve their agricultural qualities.

Homozygosity in cultivars prevents any response to selection pressures during the cultivation process. We positively require this protection if valuable agricultural characters are not to be lost. When breeding pure line crops, responses to selection pressures can occur only during the breeding process. The same is true of vegetatively propagated crops.

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