DNA Testing Science Explained
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.
A DNA test is performed by collecting DNA, typically by using a cotton swab to gather cells from the inside of an individual’s mouth. Samples are sent to a laboratory, where DNA is isolated from the rest of the cell tissue and amplified using a process called the Polymerase Chain Reaction. The polymerase chain reaction refers to a series of steps in which DNA is heated and cooled repeatedly. Heat-stable enzymes (isolated from a hot-springs-dwelling bacterium) catalyze the rapid production of billions of copies of DNA. A child’s amplified DNA can then be compared to that of the mother and the alleged father. Several genes are analyzed in order to generate a Combined Paternity Index (figure 3). Many laboratories test fifteen or more genes, and boast probability of paternity confidence levels of 99.99%. If the child does not possess any of the male’s alleles, the man will be excluded from paternity.