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In the last chapter, we saw how errors in DNA arise and what common types of error occur. What is the fate of all these mistakes? Most, as we have seen, are corrected by DNA repair mechanisms. In this case, the DNA sequence is not changed. Some errors escape these repair mechanisms and are replicated as faithfully as the original DNA sequence. If such mutations occur in somatic (body) cells, they can be passed on in the individual’s cells through mitotic cell divisions, but they will not be passed on to progeny. If they occur in the germ line, they can be passed on to progeny. Over time, through evolution (Chapters 1 and 21), the proportion of individuals in a population carrying these mutations may increase or decrease. Therefore, if we look at any present-day population of organisms, such as the human population, we will find that it harbors lots of genetic differences, all of which result from mutations that occurred sometime in the past. Genetic variation refers to genetic differences that exist among individuals in a population at a particular point in time.

In the human population, you can observe the effects of common genetic differences by looking at the people around you. They differ in height, weight, facial features, skin color, eye color, hair color, hair texture, and in many other ways. These traits differ in part because of genetic variation, and in part because of the environment. Weight is affected by diet, for example, and skin color by exposure to sunlight.

In this chapter, we consider examples of major types of DNA variation and chromosomal variation in populations today and examine their consequences for the organism. We also describe key molecular techniques that allow genetic variation to be studied directly in DNA molecules. The emphasis here is primarily, but not exclusively, on human populations.