Humans as a species are not especially diverse, genetically at least. Compared to our nearest relatives, the chimpanzees, we are quite homogeneous—that is, most of our alleles are the same within a population, and each human differs from other humans by, on average, 0.1% of their DNA. This may not seem like much, but if we have 3 billion bases in our genomes, then we expect that any two humans will differ by approximately 3 million bases. This population-level variation can make DNA testing fairly reliable. Paternity testing focuses on genes that are known to vary in the population. For each allele a child shares with the alleged father, the probability of paternity increases.