[maro]

That's not the same as the a priori probability of the allele. Here's an excerpt from a book I have never gotten around to reading, "Molecular Evolution" by Wen-Hsiung Li: "We note that a new mutant arising as a single copy in a diploid population of size N has an initial frequency of 1/2N....For a neutral mutation, P [the probability of fixation] = 1/2N." Pp. 47-48.

[Nebullis]

The initial frequency is 1/2. With neutral drift the allele carrying this allele becomes homozygous.

Good book, btw.