Passage Five Modern humans emerged some 250,000 years ago, yet agriculture is a fairly recent invention, only about 10,000 years old. Many crop plants are rather new additions to our diet: broccoli (a flowering mutant of kale) is thought to be only 500 years old. Most innovation is far more recent still. Although Austrian monk Gregor Mendel’s pea plant experiments quietly laid the basic foundations of genetics in the mid-19th century, his work was rediscovered and applied to crop breeding only at the beginning of the 20th century. Further advances have steadily accumulated. The 1940s saw the identification of DNA as genetic material and the adoption, by commercial breeders, of genetic modification - typically by applying chemicals or radiation to DNA to try to make plants with advantageous characteristics. The modifications ultimately led to the green revolution of the 1960s and 1970s, during which time global wheat yields tripled. The 1980s and 1990s saw the commercial adoption of agricultural biotechnology, which has allowed breeders to introduce specific genes into crops from the same or different species. In 2004 the first plant genome was fully sequenced, and since then the number of plant gene sequences in GenBank, the public repository for gene sequence information, has been doubling every two years. Our knowledge is increasing exponentially, as it has been in other fields such as semiconductors and cellular telephony. Our challenge is to increase agricultural yields while decreasing the use of fertilizer, water, fossil fuels and other negative environmental inputs. Embracing human ingenuity and innovation seems the most likely path. Plants did not evolve to serve humans, and their sets of genes are incomplete for our purposes. The integral role of modifying genes is obvious to all breeders, though sometimes painfully absent from the public’s understanding of how modern agriculture succeeds. All breeding techniques, from before Mendel’s time until today, exploit modifications to plant DNA. These modifications can take the form of mistakes or mutations that occur during natural cell division in the wild; the natural but random movement of DNA sequences from one part of a plant’s genome to another; or the more precise insertion of known gene sequences using biotechnology. In all these cases, plant genes are moved within or across species, creating novel combinations. Hybrid genetics - the combination of different versions of the same gene – has resulted in spectacular yield increases. Largely as the consequence of using hybrid seed varieties, corn yields in the U.S. have increased more than 500 percent in the past 70 years.Questions 21-25 are based on Passage Five.
A．The number of gene sequences has doubled since its foundation.B．The commercial breeders are its main sponsors.C．It is a genetic sequence database.D．It was founded in 2004.
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