Abstract
Kimura and Ohta showed that the expected age of a neutral muta-
tion observed t o be offrequency x in a population is -2a:(1-2)-' log x.
We put this classical result in a general coalescent process context that
allows questions t o be asked about mutations in a sample, as well as in
the population. In t h e general context the population size may vary
back in time. Assuming an infinitely-many-sites model of mutation,
we find the distribution of the number of mutant genes a t a particular
site in a sample; the probability that an allele a t that site of a given
frequency is ancestral; the distribution of the age of a mutation given
its frequency in a sample, or population; and the distribution of the
time t o the most recent common ancestor, given the frequency of a
mutation in a sample, or in the population.
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