There are two common genetic distance ： Nei Genetic Distance(Nei,1972) and Nei Unbiased Genetic Distance(Nei,1978).
Nei, M. (1972). Genetic Distance between Populations. The American Naturalist 106, 283–292.
Nei, M. (1978). Estimation of Average Heterozygosity and Genetic Distance from a Small Number of Individuals. Genetics 89, 583–590.
In the second paper, we can notice the difference or know why we need the unbiased distance:
The magnitudes of the systematic biases involved in sample heterozygosity and sample genetic distances are evaluated, and formulae for obtaining unbiased estimates of average heterozygosity and genetic distance are developed. It is also shown that the number of individuals to be used for estimating average heterozygosity can be very small if a large number of loci are studied and the average heterozygosity is low. The number of individuals to be used for estimating genetic distance can also be very small if the genetic distance is large and the average heterozygosity of the two species compared is low.
I think we usuall have less than 50 samples in each population， so Nei Unbiased Genetic Distance is recommended.
Our formulae for obtaining unbiased estimates of average heterozygosity and genetic distance apply to any sample size and are superior to sample average heterozygosity and genetic distance, as long as many loci are used. However, the difference between the biased and unbiased estimators is very small when the number of individuals used is large, say more than 50.