DNA is extracted and purified (from the buccal swabs, blood, etc.) using an enzyme which releases it into solution. To determine sizes of DNA in a particular region the DNA region is either:

• amplified using DNA primers, targeted to the regions, and a DNA polymerase enzyme (PCR method), or
• cut with restriction enzymes (RFLP method).

The DNA samples are then ran through a system of capillaries using a chromotography method (called capillary electrophoresis) to resolve sizes. An example of an older procedure called gel electrophoesis is shown in Figure 1 to assist in illustrating the separation effect of chromotography. A computer-generated electropherogram (Figure 2) demonstrates how the DNA sizes can be obtained using a fluorescent detection system. It is similar to reading a barcode. This procedure has replaced the older gel electrophoesis method illustrated in Figure 1.

To visualize the DNA, either:

• the primers are labeled before the PCR reaction (PCR method), or
• a labeled probe is used to hybridize to the specific region of DNA being examined (RFLP method).

Each person should have 2 alleles at each system examined for paternity testing (with exception given to either, a) a person having the same allele presented twice, or b) the possibility of a 'silent' allele). One allele is passed from the biological father and the other allele is passed from the biological mother.

Each known allele has a determined frequency of occurrence in the general population and among different ethnic groups (Table 1). These frequencies are used to calculate the probabilities of paternity, siblingship, grand-parentage, etc.

If you have any questions regarding this process please let us know and we will be happy to assist you with answering them.

Figure 2: An electropherogram (or chromatograph) showing the peaks that represent the 2 alleles established at each chromosomal location tested for one individual (i.e., D13S317, D7S820 and D16S539). In this case, the data collected is that from Lane 2 (Child) in Figure 1

.

Figure 1: An autoradiograph representing genomic DNA sample detected using silver staining. Lanes labeled L contain allelic ladders for the respective loci. Numbers to the right of the image indicate the alleles established for the samples ran in lanes 1 (Alleged Father) and 2 (Child).

Table 1: Allele Frequencies for Caucasian-Americans (N = number of alleles observed and AF = allele frequency).