Bacterial Mutations Mutations—heritable changes in DNA—occur spontaneously, but breeders can increase the mutation rate of an organism by using radiation or chemicals. Many mutations are harmful to the organism. With luck and perseverance, however, breeders can often produce a few mutants—individuals with mutations—with useful characteristics that are not found in the original population. This technique has been particularly useful with bacteria. Because they are small, millions of bacteria can be treated with radiation or chemicals at the same time, which increases the chances of producing a useful mutant. This technique has allowed scientists to develop hundreds of useful bacterial strains. For instance, we have known for decades that certain strains of oil-digesting bacteria are effective for cleaning up oil spills. Today scientists are working to produce bacteria that can clean up radioactive substances and metal pollution in the environment.

Polyploid Plants Drugs that prevent the separation of chromosomes during meiosis are very useful in plant breeding. These drugs can produce cells that have many times the normal number of chromosomes. Plants grown from these cells are called polyploid because they have many sets of chromosomes. Polyploidy is usually fatal in animals. But, for reasons that are not clear, plants are much better at tolerating extra sets of chromosomes. Polyploidy can quickly produce new species of plants that are larger and stronger than their diploid relatives. A number of important crop plants, including bananas and many varieties of citrus fruits, have been produced in this way. Figure 15–4 lists several examples of polyploid plants.

FIGURE 15–4 Ploidy Numbers Because polyploid plants are often larger than other plants, many farmers deliberately grow polyploid varieties of crops like those listed above. Interpret Tables Which plant has undergone the most dramatic changes in chromosome number?

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15.1 Assessment

Review Key Concepts

1. 1. Review Give an example of selective breeding.

   2. Compare and Contrast Suppose you are a geneticist trying to develop a sunflower with red petals and a short stem. As you compare the sunflowers you have on hand, what genetic variations would you look for? What kinds of plants would you select for crossing?

2. 1. Review What is the relationship between genetic variations and mutations?

   2. Explain How can breeders introduce mutations?

   3. Draw Conclusions How is selective breeding a form of biotechnology?

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Explanation

3. Write a paragraph in which you suggest ways that plants could be genetically altered to improve the world's food supply. (Hint: The first sentence in your paragraph should express the paragraph's main idea.)

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