DNA Testing Resources
Overview
Non-standard samples
Chain of Custody
Results Interpretation
DNA 101
Overview
How is Nuclear DNA Organized?
Why is Autosomal DNA Ideal for Identity and Parentage Testing?
What is the basis for autosomal STR testing?
Steps in DNA Sample Processing
Raw STR Data
Interpreting STR Results in a Paternity Test
Y-STR Analysis
Mitochondrial DNA Analysis
DNA Glossary
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Orchid Cellmark holds multiple accreditations from various organizations and adheres to the standards of each accrediting organization. Compliance by our laboratories to these accreditation requirements provides assurance to our clients that test results provided by Orchid Cellmark are of the highest quality. Clink on the links above to learn more about our accreditations.
DNA Testing Resources
DNA 101
Interpreting STR Results in a Paternity Test
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.
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, a Paternity Index (PI) is calculated for each STR system. This PI is a comparison of obtaining these results if the alleged father is the true father to obtaining these results from a man who is not the father. The calculation of this PI is dependent on the race or ethnicity of the alleged father as it is performed using gene frequencies obtained from population data. Since all the STR systems are inherited independently of one another the PI for each system can be multiplied by the PI’s for all STR systems tested to obtain a Combined Paternity Index (CPI). This CPI can then be converted into a probability of paternity. For example, if the CPI equals 644,144, the probability of paternity is 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 gametes (in this case sperm) so that the child’s DNA appears not to match that of his/her biological father or mother at a particular loci.