The amount and pattern of DNA polymorphism can give useful information on the maintenance mechanism of genetic variation at the DNA level. In this note we have shown the amount and pattern of DNA polymorphism expected under the neutral theory. The amount of DNA polymorphism can be estimated from the average number of nucleotide differences per site, the proportion of segregating sites, and so on. We have shown how to estimate theta from these quantities, where theta = 4Nv, N is the effective population size and v is the mutation rate per site per generation. We have also shown the expectations of the nucleotide variation within and between allelic classes.