91 lines
5.5 KiB
Plaintext
91 lines
5.5 KiB
Plaintext
ÜÜÜÜÜÜÜÜÜÜÜÜÜ ÜÜÜ ÜÜÜÜ
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ÜÛÛÛÛÛÛÛÛßÛßßßßßÛÛÜ ÜÜßßßßÜÜÜÜ ÜÛÜ ÜÛÛÛÛÛÛÛÛÜÜÜÜÜÛßß ßÛÛ
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ßÛÛÛÛÛÛÛÛÛÛÛÛÛÛÜ ßÛÛ ÜÛÛÛÜÛÛÜÜÜ ßÛÛÛÛÜ ßÛÛÛÛÛÛÛÜÛÛÜÜÜÛÛÝ Ûß
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ßßßÛÛÛÛÛÛÛÛÛÛÜ ÞÝ ÛÛÛÛÛÛÛÛÛÛÛßßÛÜÞÛÛÛ ÛÛÛÛÛÜ ßßÛÛÛÞß
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Mo.iMP ÜÛÛÜ ßÛÛÛÛÛÛÛÝÛ ÞÛÛÛÛÛÛÛÛÛ ÞÛÛÛÛ ÞÛÛÛÛÛÝ ßÛß
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ÜÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÝ ÞÛÛÛÛÛÛÛÛÝ ÛÛÛ ÛÛÛÛÛÛ
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ÜÛÛÛÛÛÛÛÝ ÞÛÛÛÛÛÛÛÛ ÞÛÛÛÛÛÛÛÛ ß ÞÛÛÛÛÛÛÜ ÜÛ
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ÜÛÛÛÛÛÛÛÝ ÛÛÛÛÛÛÛÛ ÛÛÛÛÛÛÛÛÝ ÞÞÛÛÛÛÛÛÛÛÛß
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ÜÛßÛÛÛÛÛÛ ÜÜ ÛÛÛÛÛÛÛÛÝ ÛÛÞÛÛÛÛÛÝ ÞÛÛÛÛÛÛßß
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ÜÛßÛÛÛÛÛÛÜÛÛÛÛÜÞÛÛÛÛÛÛÛÛ ÞÛ ßÛÛÛÛÛ Ü ÛÝÛÛÛÛÛ Ü
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ÜÛ ÞÛÛÛÛÛÛÛÛÛÛß ÛÛÛÛÛÛÛÛÛ ßÛÜ ßÛÛÛÜÜ ÜÜÛÛÛß ÞÛ ÞÛÛÛÝ ÜÜÛÛ
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ÛÛ ÛÛÛÛÛÛÛÛß ÛÛÛÛÛÛÛÛÛÛÜ ßÛÜ ßßÛÛÛÛÛÛÛÛÛß ÜÜÜß ÛÛÛÛÜÜÜÜÜÜÜÛÛÛÛÛß
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ßÛÜ ÜÛÛÛß ßÛÛÛÛÛÛÛÛÛÛÜ ßßÜÜ ßßÜÛÛßß ßÛÛÜ ßßßÛßÛÛÛÛÛÛÛßß
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ßßßßß ßßÛÛß ßßßßß ßßßßßßßßßßßßß
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ARRoGANT CoURiERS WiTH ESSaYS
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Grade Level: Type of Work Subject/Topic is on:
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[ ]6-8 [ ]Class Notes [Report on breeding ]
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[x]9-10 [ ]Cliff Notes [plants." ]
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[ ]11-12 [x]Essay/Report [ ]
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[ ]College [ ]Misc [ ]
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Dizzed: 07/94 # of Words:630 School:Prep/Boys State:NY
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ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>Chop Here>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ>ÄÄÄÄÄÄÄÄÄ
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PLANT BREEDING
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Mankind has selectively bred plants for thousands of years with the
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aim of improving their quality. Experimenting with cereal crops, breeders
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have aimed to enhance their yield of grain, the quality of their flour, and
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their resistance to disease and drought. With other plants, breeders have
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tried to improve the perfume and color of the flowers.
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Improvement of plants, particularly food crops, is obviously important
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and genetics has contributed to a better understanding of the benefits and
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disadvantages of particular breeding programs. Many cereal crops such as
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corn are now planted largely as hybrid seed, produced by outbreeding
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between different inbred varieties. The vigour of the hybrid plant is
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probably a major contribution to the increased corn output in the United
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States. This increased output represents a major achievement for applied
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genetics. In 1929 practically no hybrid corn was grown among the 100
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million acres of corn in the United States. But by 1970 the vast
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majority of 67 million acres was planted with the hybrid variety, yielding
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twice as much corn.
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Plant breeders have a definite advantage over animal breeders, because
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they can often produce fertile varieties - indeed, new species - by
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crossbreeding between species. This is because hybrids are often polypoid.
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Polyploidy, as we know, can occur naturally in the wild. Some species of
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cotton that we grow are polypodies that probably arose originally by
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accidental crosses between different species of cotton.
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But breeders do not have to rely on accidents. They can attempt to
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produce fertile polypodies by crossbreeding between different species. One
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early attempt to produce another hybrid species was made in 1927 by the
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Russian geneticist G. D. Karpechenko, who crossbred two quite distantly
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related species, a radish and a cabbage. Each species has eighteen
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chromosomes (nine pairs); the hybrids had the same number (nine radish
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chromosomes and nine cabbage chromosomes) and were sterile. However, some
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polyploids arose by chance. These had thirty six chromosomes (nine pairs
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of radish, and nine pairs of cabbage), and were fertile. Unfortunately,
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the hybrid was not commercially successful because as luck would have it,
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the plant had the leaves of a radish and the roots of a cabbage!
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Breeders can artificially encourage polyploidy by treating the hybrids
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that result from crossbreeding between species with a chemical called
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colchicine obtained from autumn crocuses. This chemical allows the
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chromosomes to reproduce, but prevents the formation of two separate cells.
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The number of chromosomes in the nucleus is therefore doubled. Several of
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these new polyploid varieties promise to be very useful. For example, a
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new hybrid cereal, called triticale, produced by crossbreeding rye with
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species of wheat, adds rye's resistance to cold winters to the usual
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properties of wheat.
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Recent breeding programs have led to highly inbred wheats. Much of the
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genetic variability, that accumulated over nine thousand years of wheat
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cultivation, is missing from present day varieties. If a new disease
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should arise, or if the climate were to change suddenly, much of the wheat
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might be damaged and lost. So it is a good idea to introduce other genes
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into wheat by outbreeding. One way of doing this is to crossbreed the
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inbred varieties with their wild relatives, which may be resistant to
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viruses, insects, or drought. For this reason, some wheat breeders believe
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it is essential to conserve some stocks of primitive wheat in seed banks,
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from which they will be able to take a transfusion of genes, if and when
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the need arises.
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Varieties of the same species of the cabbage family have been
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selectively cultivated for their differing features of taste and
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appearance. Some varieties became hard headed, like a modern cabbage, some
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made masses of flower buds, as in cauliflower and broccoli, and some made
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clusters of leaf buds, as in Brussels sprouts.
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