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[[Image:DNA Overview.png|thumb|right|A section of [[DNA]]]]
Every living organism contains molecules of DNA, which carries genetic information. [[Genes]] are the pieces of DNA that carry this information and they influence the properties of an organism. Genes determine a person's general appearance and to some extent their behavior. Since close relatives have similar genes they tend to look alike.
If two organisms are closely related, their DNA will be very similar.<ref name= NAS>{{cite web |url=http://www.nap.edu/readingroom/books/evolution98/evol3.html |title= Teaching about evolution and the nature of science |accessdate=2007-12-30 |last= Kennedy |first=Donald |coauthors=(Working group on teaching evolution) |date=1998 |work=Evolution and the nature of science|publisher= The National Academy of Science }}</ref> On the other hand, the more distantly-related two organisms are, the more differences they will have. For example, two brothers will be very closely related and will have very similar DNA, while distant cousins will have far more differences between their DNA. As well as showing how closely-related two individuals are, DNA can show how closely-related two species are. For example, comparing [[chimpanzee]]s with [[gorilla]]s and humans shows that there is as much as a 96 percent similarity between the DNA of humans and chimps, and that humans and chimpanzees are more closely related to each other than either species are to gorillas.<ref>{{cite web |url= http://news.nationalgeographic.com/news/2005/08/0831_050831_chimp_genes.html |title=Chimps, Humans 96 Percent the Same, Gene Study Finds |accessdate=2007-12-23 |last= Lovgren |first= Stefan |coauthors= |date=2005-08-31 |work= National Geographic News |publisher= National Geographic }}</ref><ref>{{Harv |Carroll|Grenier|Weatherbee|2000| p=}}</ref>
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