Possible problems The use of mitochondrial mtDNA to investigate human history is not without drawbacks. The rate of mtDNA mutation is not well known. A study by Parsons et al. (1997) found a rate 20 times higher than that calculated from other sources. In an article reviewing mtDNA research, Strauss (1999a) reports that mtDNA mutation rates differ in some groups of animals, and can even vary dramatically in single lineages. Although there are many agreements, some divergence dates for modern animals calculated from mtDNA do not match with what is known from the fossil record. There are suggestions from a few sources that paternal mtDNA can sometimes be inherited, which could affect analyses based on mtDNA. In 1999 Awadalla et al. published a study suggesting that mtDNA could sometimes be inherited from fathers. If mtDNA is inherited only from mothers, the correlation between different mutations should not depend on how far apart on the genome they were. Instead, their measurements showed that mutations at distant sites on the mtDNA genome were less likely to
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