For example, Jersey cows produce very creamy milk in small amounts; Friesian cows produce large quantities of milk. Jersey cows producing the creamiest milk are bred with the male offspring of Friesian cows producing the most milk. The offspring cows produce large amounts of creamy milk. These are then bred together and the process repeated until there is a herd of cows producing large amounts of very creamy milk.
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Plant growers also use selective breeding. They transfer pollen from one flower to another to breed plants with specific characteristics. 3. What characteristics may be desirable in breeding roses and tomatoes? 4. Explain how a tomato grower would selectively breed tomatoes for a characteristic you identified in question 3.
Why is selective breeding important?
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FIGURE 3.1.5c: Why would a farmer breed a Jersey bull with a Friesian cow?
Selective breeding plays a major role in supplying the Earth’s population with food. The development of plant and animal breeds has allowed plant growing and animal farming to become an industry, for example producing: •
short-stemmed varieties of grain crops suitable for combine harvesting
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vegetable varieties suitable for cultivation in greenhouses
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grapes and tomatoes suitable for machine harvesting
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cattle adapted to maintenance in large livestock buildings.
There are drawbacks to selective breeding. The genetic variation in the population is reduced. If there is a change in the environment, such as a new disease, the population may not be able to survive. Also, many animals produced using selective breeding suffer from deformities. The selectively bred Belgian Blue cattle have a number of problems: •
difficulty with calving (Caesarean births are common)
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jaw deformities and over-enlarged tongues, which mean that calves cannot suckle and may die
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heart and respiratory (breathing) problems.
5. How has selective breeding of plants impacted on farming? 6. Do the advantages of selectively breeding animals outweigh the drawbacks?
FIGURE 3.1.5d: Belgian Blue cattle have been selectively bred for meat production.
Key vocabulary selective breeding
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