Caveats About Sequence Comparisons
On the other hand, the simplicity and power of sequence comparisons can lead too easily to an oversimplified interpretation of results, and to conclusions that may sound more significant than they are. A prime example is the often-repeated statement that humans and chimpanzees share 98 percent of their DNA.
It may be true that 98 out of 100 bases are the same in the two genomes, but what is the significance of this fact? It does not mean that 98 percent of our genes are identical. In fact, almost all of them differ slightly, some dramatically. It also does not tell us whether the significant differences between humans and chimps arise from a few very different genes, or many slightly different ones. Moreover, there are significant differences in genome structure not accounted for by the sequence comparison. For instance, humans have forty-six chromosomes, whereas chimpanzees have forty-eight; they have about 10 percent more DNA than humans do; and humans have more copies of a certain kind of transposable genetic element than they do.
Most importantly, the sequence similarity certainly does not tell us that humans "are" 98 percent chimpanzee—we are two entirely different species, as is obvious from differences in anatomy and behavior. If the profound differences between humans and chimps are not reflected in the sequence data, it may be that this simple tabulation of difference does not adequately summarize the ways in which DNA can cause two organisms to differ.
The 98 percent figure, therefore, may be used to say that chimps and humans are closely related, and are more closely related to each other than either is to an organism with a greater number of sequence differences, such as the orangutan. However, it may not be used to draw conclusions about the similarity of humans and chimps as organisms.
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