This is a note for the book Population Genetics: A Concise Guide (Second Edition).
Page 30
Problem 2.7 How many different alleles are one mutational step away from an allele at a locus that is 3000 nucleotides in extend? How many are two mutational step away?
Note:
One nucleotide has one of the three possibilities of mutation: A->{T,C,G}, T->{A,C,G}, C->{A,T,G}, G->{A,T,C}. So for an allele at a locus that is 3000 nucleotides in extend has 3000*3=9000 alleles which are one mutational step away.
For two mutational step away: for the first mutation position, there are 3000 possibilities and for the second position position, there are 2999 possibilities, so altogether there are 3000*3*2999*3=80973000 alleles.