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Genetic distance is a measure of the dissimilarity of genetic material between different species or individuals of the same species.

All of life today is based upon the molecule of inheritance, DNA (deoxyribonucleic acid). DNA molecules are a double helix, similar to a spiral staircase, in which the backbone sides are comprised of alternating 5-carbon deoxyribose sugar and a phosphate molecule, whilst the "rungs" of the staircase are nitrogenous purine and pyrimidine bases. These bases are adenine (A), which always pairs with thymine (T), and guanine (G), which always pairs with cytosine (C). It is the sequence of these bases on the helix, (e.g. ATTTCGCCAAG) which is copied and passed to descendants in cellular reproduction.

By comparing the percentage difference between the same genes or junk DNA of different species, a figure can be obtained, which is a measure of "genetic distance". Depending upon the difference, and correcting this for known rates of evolution, genetic distance can be used as a tool to construct cladograms showing the family tree of all living beings.

For instance, the fact that the genetic distance of chimpanzees and human beings is only 1.6% (they are about 98.4% identical), suggests that human beings and chimpanzees last had a common ancestor about 5 million years ago, and that chimpanzees and humans are more closely related than either of the two species are related to gorillas and orangutan (which diverged about 9 million years ago, and 12 million years ago, respectively).

There are several different methods for defining genetic distance. One genetic distance measure is given by the formula D=−log(In). The quantity In is called the "genetic identity" or "genetic similarity", and defined as In=ΣI÷#loci, where I=(Σ Pix•Piy)÷[(ΣPix²)·( ΣPiy²)]½. Pix is the proportion of allele i in population X, Piy is the proportion of allele i in population Y.

• A review of mutation processes and methods of phylogenetic inference, David B. Goldstein, David D. Pollock
• Stanford University website on genetic distance
• The Estimation of Genetic Distance and Population Substructure from Microsatellite allele frequency data., Brent W. Murray (May 1996), McMaster University website on genetic distance
• Computing distance by stepwise genetic distance model, web pages of Bruce Walsh at the Department of Ecology and Evolutionary Biology at the University of Arizona

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