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Deciphering Paternity Test Outcomes: The Role of 21 STR Markers Explained

Navigating through the realms of DNA and genetic testing, paternity tests stand out as a pivotal tool for uncovering biological ties. A key method employed in these tests is the analysis of Short Tandem Repeats (STRs). This article aims to simplify the concept and interpretation of paternity test results that utilize 21 STR markers, making the data accessible and understandable.

1. Introduction to STRs

Short Tandem Repeats (STRs) are sections within DNA where a sequence of nucleotides is repeated. The variability of these sequences from one individual to another renders them perfect for DNA matching purposes. The term “21 STR markers” refers to the examination of 21 unique and variable spots within the genome during a paternity test.

2. Why 21 STR Markers?

The choice of 21 specific STR markers in paternity testing is driven by their genetic diversity across populations. This diversity ensures the precision and reliability of the test, where matching markers between the alleged father and the child strongly indicate a biological connection.

3. Reading the Results

The paternity test report typically presents a comparison chart of STR markers for the child, mother, and alleged father. Here’s a breakdown of what the key columns represent:

4. Matching and Mismatching Alleles

At each marker, the child inherits one allele from each parent. If the child’s allele at any locus does not match the mother’s, the corresponding allele should match one from the alleged father for him to be considered the biological parent.

5. Probability of Paternity Indicator

The test results conclude with a “Probability of Paternity” percentage. A probability over 99.9% generally affirms paternity, while a 0% outcome negates the biological relationship.

6. The Combined Paternity Index (CPI)

The CPI quantifies the likelihood of paternity by comparing the genetic makeup of the alleged father with that of a random male from the population. Higher CPI values reinforce the probability of paternity.

7. Contextual Considerations

While the analysis of 21 STR markers yields highly accurate insights, definitive paternity conclusions should also account for broader evidence and legal standards. For legal cases, maintaining a documented chain of custody for the samples is crucial to validate the test results.


Interpreting paternity test results based on 21 STR markers need not be overwhelming. A fundamental understanding of STRs, alleles, and the layout of the report demystifies the process. This scientific approach provides clear answers to significant familial questions. For any uncertainties, consulting with the DNA testing service or a genetic counseling expert is advisable to gain further insights.



About DNAForce Inc.: Established in 2008, DNAForce Inc. stands as a beacon of trust and accuracy in the field of DNA testing. With over a decade of experience, we've proudly assisted thousands in their quest for genetic clarity, boasting over 15,000 tests sold. Whether it's paternity, maternity, or kinship testing, our commitment to precision has made us a top choice for those seeking answers in the intricate maze of DNA. When you choose DNAForce Inc., you're choosing a legacy of excellence, accuracy, and unparalleled customer trust.

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