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Fixation Index

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# Fixation Index

The fixation index is the average coefficient of inbreeding in a population. In case of random mating, the probability that an offspring would have exactly the same two ancestral alleles at a locus is (1/2)N, where N is the number of diploid individuals in the population. The probability of having two different alleles at the same locus is 1 − (1/2)N. The coefficient of inbreeding of the first generation of this population is also (1/2)N by definition of inbreeding. In each succeeding generation, the non-inbred part of the population will have a chance to produce offspring with an allele pair identical by descent. Therefore, the coefficient of inbreeding in the next generations will be (1/2)N + [(1 − (1/2)N] x F, where F is the inbreeding coefficient of the preceding generation. After the gth generation the coefficient of inbreeding of this population will be: F

_{g}= (1/2)N + [1 − (1/2)N]F_{g−1}and this is called the*index of fixation*. Its complement is the panmictic index