Since the dawn of agriculture some 10,000 years ago, farmers have been developing varieties of plants to yield desired results. When you eat an ear of corn, for example, you can thank the prehistoric agriculturalists who started the process by selecting types of maize that retained their seed on the ear, leading to the development of corn on the cob. Variety development can be accomplished through many activities, from simply selecting plants with desirable characteristics for propagation to more complex molecular breeding techniques.
Plant breeding is the generation of variation, selection of plants and genetic stabilization (fixing) of traits to obtain varieties with reproducible desired characteristics. Its scientific underpinnings began to be understood in the mid-nineteenth century with the work of Gregor Mendel, the father of genetics. Using the pea plants in his garden, Mendel observed how traits were passed down to succeeding generations and he formulated the idea that specific traits were inherited as units in a predictable way. By the mid-twentieth century, scientists had established that traits are transmitted by genes in chromosomes, which store and express chemical information resulting in these characteristics. An understanding of genetic principles and their application to plant breeding technology has greatly accelerated the rate of improvement of crop plants. Researchers estimate that at least half of the several-fold yield increases attained in wheat and rice during the Green Revolution resulted from the development and use of genetically improved varieties.
Plant traits are encoded in the DNA of their genes. Sometimes many different genes can influence a desirable trait, making it difficult for plant breeders to accumulate them all into a single variety. Marker-assisted breeding allows breeders to map and trace thousands of genes and screen large populations of plants for those that possess the traits of interest. The marker, or genetic tag, can be based on either DNA or proteins. Molecular markers have enabled high-throughput genotyping and accelerated the rate at which breeders can incorporate useful traits into new varieties.
Breeders often make crosses between plants of diverse genetic makeup or genotypes to produce new combinations of genetic traits, which then result in diverse phenotypes, or observable morphological or quality traits in the progeny plants. The natural diversity of different sources of germplasm within a species or its close relatives is a primary source of genetic variation. Genetic variation can also be increased by inducing mutations, changes in the DNA sequences of the plants. Since the 1950s, over 2,200 crop varieties have been developed by induced mutations. In 1973, it became possible to identify and splice (or recombine) specific DNA molecules, leading to recombinant DNA technology or genetic engineering, which allows scientists to copy and exchange genes among species to introduce new characteristics, such as resistance to herbicides (compounds that control weeds) or insects. Plants developed using genetic engineering are often called transgenic plants.
Green Revolution: Advances in genetics, crop protection, fertilizers and machinery that culminated in dramatic increases in crop productivity during the third quarter of the 20th century.
Genotype: the total of all genetic information contained in an organism, regardless of whether it is evident in the observable or measureable traits (the phenotype).
Phenotypes: Observable or measureable characteristics of an organism that result from interactions of its genetic constitution (its genotype) with the environment in which it grows.