McDougal Littell Science Cells and Heredity 

Table of Contents
Page 150 (continued)

Changes in DNA can change an organism.

Organisms change over time. Changes come about because of mutations in DNA. Random changes in DNA may introduce new traits into an organism. Over time, certain traits may become more common in one group of organisms as they interact with the environment and each other.

Are all changes in a group of organisms random? There are dogs, such as bloodhounds, that are particularly well suited to tracking. There are cows that give large quantities of milk and crops that produce large quantities of grain. Changes such as these are not random, but result from careful breeding directed by humans.

Page 151

Selective Breeding

For thousands of years, humans have been carefully selecting and breeding certain plants and animals that have desirable traits. As the years have passed, horses have gotten faster, pigs have gotten leaner, and corn has become sweeter. Selective breeding is the process of selecting and breeding parent organisms to pass on particular traits to the offspring.

Selective breeding can be successful as long as the desirable traits are controlled by genes. In fact, what these early farmers were actually selecting were alleles, particular versions of a gene. The alleles were already present in some members of the population. People were not changing DNA, but they were causing certain alleles to become more common in a particular breed. The different dog breeds are a good example of this. All dogs share a common ancestor, the wolf. However, thousands of years of selective breeding have produced dogs with a variety of characteristics.

IdeaWeb

Bloodhounds, with their strong sense of smell, are used in police work for tracking.

Arrow
CHECK YOUR READiNG

How does selective breeding affect DNA?

Genetic Engineering

Within the last fifty years it has become possible to directly change the DNA of an organism. Genetic engineering is the process in which a sequence of DNA from an organism is first isolated, then inserted into the DNA of another organism, changing that organism's DNA. The DNA that is engineered often codes for some particular trait of interest. Using technology, scientists can take a gene from one species and transfer it into the DNA of an organism from another species. The resulting organisms are referred to as genetically modified (GM), or transgenic.

Arrow
CHECK YOUR READiNG

What are three steps involved in genetic engineering?

One application of genetic engineering across species involves making plants more insect-resistant. Genetic engineers have isolated genes in microorganisms that produce natural insect-killing chemicals, or pesticides. They have succeeded in transferring these genes into the DNA of crop plants, such as corn and soybeans. The cells of the genetically modified plants then produce their own pesticide, reducing the amount of chemical pesticide farmers need to use on their fields.

Page 152
Tomato

These tomatoes have been genetically modified to grow in conditions that would not support naturally occurring tomatoes.

Genetic engineering can address very specific needs. For example, in many parts of the world, soils are poor in nutrients. Or the soil may contain salts. Such soil is not good for growing food crops. Genetic engineers have inserted a gene from a salt-tolerant cabbage into tomatoes. The salt-tolerant tomatoes can grow in soil that natural tomatoes cannot grow in. These tomatoes can also be grown using brackish water, which is water with a higher salt content than fresh water.