Raw STR data can also be visualized in a linear fashion, similar to a “boxcar” of a train. Each “boxcar” in the diagram below represents a copy of the STR of interest.

By comparing the STR profile of the child, first to the mother, it is possible to determine which of the child’s alleles came from the mother.

In the “Inclusion” example below, the child and mother both have the STR allele that is repeated 5 times. That means that the child’s STR allele that is repeated 3 times, comes from the child’s biological father. One then determines if the alleged father has the STR that is repeated 3 times. If so, this man is a possible father.

In the Exclusion example below, the child’s maternal allele has already been determined to be the “5” and the child’s paternal allele is the “3”. One then examines the alleged father’s DNA profile and since it does not contain a “3”, this is considered an exclusion.

Each STR region is examined in the manner described above. If, at the end of the test, the laboratory has observed only inclusions between the child and alleged father, that matching DNA profile is compared to a population database on individuals of the same race to determine how common that profile is in the general population. The frequency of that profile can then be converted into a probability of paternity. For example, if the STR profile of the alleged father is found 644,144 times in the general population, this is equivalent to a probability of paternity of 99.99%.

If, at the end of the test, the DNA of the child and alleged father show exclusions at 3 or more loci, then the alleged father is considered to be excluded as the biological father of the child. In this case, the probability of paternity would be 0.00%. Exclusions at 1 or 2 loci are not considered enough to conclude that the alleged father is not the biological father because there are certain instances when mutations take place during the formation of the child so that the child’s DNA appears not to match that of his/her biological father.