Genetic variability is prevalent in plants and other organisms that reproduce sexually and thereby produce spontaneous mutants. Throughout most of history, plant domestication and breeding were primarily based on the propagation of mutants. When a grower observed a plant with a potentially desirable mutation(such as a change that produced bigger fruit, brighter flowers, or increased insect resistance), the grower would collect seeds or take cuttings and produce additional plants with the desirable characteristic. Advances in the understanding of genetics in the early part of the twentieth century made it possible to breed some of the desirable characteristics resulting from mutation into plants that previously had lacked the characteristic.
The obvious advantages of producing plants with improved characteristics such as higher yield made plant breeding very desirable. As human populations continued to grow, there was a need to select and produce higher-yielding crops. The development and widespread use of new high-yield varieties of crop plants in the 1960's is often referred to as the Green Revolution. Basic information supplied by biological scientists allowed plant breeders to fuse a variety of characteristics from different plants to produce new, higher-yielding varieties of numerous crops, particularly seed grains.
When a plant characteristic is identified as desirable, it is studied both morphologically and biochemically to determine the mechanism of inheritance. If it is determined that the mechanism is transferable, attempts are made to incorporate the trait into the target plant. If the plants are closely related, traditional breeding techniques are used to crossbreed the plant with the desirable trait with the plant that lacks the characteristic. Although this process is often tedious, it is based on a fairly simple concept. Basically, pollen from one of the plant types is used to fertilize the other plant type. This process often requires specialized handling techniques to ensure that only the pollen from the plant with the desired characteristic is allowed to fertilize the eggs of the recipient plant.
Sometimes this process involves the use of bags or other materials to isolate the recipient flowers, which are then pollinated by hand. Another technique involves the introduction of a gene for male sterility into the recipient plant. In these cases, only pollen from another plant can be used to fertilize the egg. Once plants with the desirable characteristics are developed, the lines are often inbred to maintain large numbers of progeny with the desired traits. In many cases, inbred lines will lose vigor after several generations. When this occurs, two inbred lines may be crossed to produce hybrids. A majority of the hybrid offspring will still contain the desired characteristics but will be more vigorous.
See also: Early Crop Domestication, Recombinant Technology
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