How DNA Paternity Testing Actually Works (The Science)

DNA inheritance basics

Every human cell has 46 chromosomes, organized in 23 pairs. For each pair, one chromosome came from the biological mother and one from the biological father. This is the basic setup that makes paternity testing possible.

The chromosomes contain DNA, which is the genetic instruction book for building and operating the body. Most of the DNA is shared across all humans (because we are all the same species), but specific regions vary from person to person. These variable regions are what paternity testing actually examines.

STR analysis: the foundation of modern paternity testing

The technique used in virtually all modern DNA paternity testing is called STR analysis, which stands for Short Tandem Repeat. STRs are short sequences of DNA that repeat themselves multiple times in a row at specific locations on the chromosomes.

For example, at a location called D3S1358 on chromosome 3, the DNA reads "TCTA" repeated multiple times. Different people have different numbers of repeats at that location. One person might have 14 repeats. Another might have 17. A third might have 13 on one chromosome and 16 on the other.

The number of repeats at each STR location is highly variable between individuals but is inherited from parents predictably. A child has exactly the same number of repeats at each location as one of their two biological parents.

The 16 to 24 markers tested

A standard modern DNA paternity test analyzes between 16 and 24 STR locations (called markers or loci). Each marker is at a specific known position on a specific chromosome. The common marker panels include locations like:

• D3S1358
• VWA
• D16S539
• D2S1338
• D8S1179
• D21S11
• D18S51
• D19S433
• TH01
• FGA
• D5S818
• D13S317
• D7S820
• And several others depending on the lab's panel

These specific markers are used because they are highly variable in the general population (which gives the test its statistical power) and because their inheritance patterns are well understood.

How matching works at each marker

At each marker, the lab measures the number of STR repeats on each chromosome for each participant. The child has two values (one from each chromosome of the pair). Each parent also has two values.

For the alleged father to be the biological father, at every single marker:

• One of his two values must match one of the child's two values

If even one marker fails this matching test, the alleged father is excluded as the biological father. There is no way around it. A man cannot pass on DNA he does not have.

If all markers match, the alleged father is included as a possible biological father. The strength of the inclusion (the probability of paternity percentage) depends on how rare the matching values are in the general population.

The mother's role

When the mother's DNA is included, the lab can identify which of the child's values at each marker came from her. Whatever remains must have come from the biological father.

This makes the comparison to the alleged father slightly more precise. The lab can confirm that the alleged father's contribution matches what was left after subtracting the mother's contribution.

Without the mother, the lab knows the child has two values at each marker but does not know which came from which parent. The test still works (any matching variant can be the paternal one), but the statistical confidence is slightly lower. The difference is typically 99.99 percent (with mother) vs. 99.9 percent (without mother), both well above the legal threshold.

How the lab generates a result

Once the lab has the STR data for each participant, the calculation proceeds in three steps:

1. Check for any exclusion. If the alleged father lacks a required variant at any marker, the result is exclusion (0 percent probability of paternity), and the calculation stops there.
2. Calculate the Paternity Index (PI) at each marker. For each matching marker, the lab computes how much more likely the observed match is if the alleged father is the biological father vs. if a random unrelated man is the biological father. This calculation depends on how common the matching variant is in the general population.
3. Combine the PI values into a Combined Paternity Index (CPI). The individual PIs are multiplied together. The CPI is then converted to a probability of paternity using a standard statistical formula.

For a typical inclusion, the CPI is in the tens of thousands or higher, and the corresponding probability is 99.99 percent or higher.

Why AABB accreditation matters

The science of STR analysis is straightforward. What is harder is doing it consistently, accurately, and with proper quality controls. That is what laboratory accreditation is for.

AABB accreditation (formerly known as the American Association of Blood Banks) is the gold standard for paternity testing labs in the United States. AABB-accredited labs:

• Use validated STR analysis methods
• Maintain documented chain-of-custody procedures
• Run regular proficiency testing to verify accuracy
• Have qualified laboratory directors overseeing all testing
• Are subject to regular inspections and re-accreditation

Courts and government agencies require AABB accreditation (or equivalent) for paternity testing because it provides assurance that the result is reliable. Non-accredited labs may produce accurate results, but there is no external verification that they consistently do so.

The actual lab process timeline

From the moment samples arrive at the AABB-accredited laboratory:

1. Day 1: Samples are logged in, verified against chain-of-custody documentation, and DNA is extracted from the buccal swabs
2. Day 1-2: The extracted DNA is amplified using PCR (polymerase chain reaction) to create enough material to test
3. Day 2-3: The amplified DNA is analyzed on a genetic analyzer to determine the STR repeat numbers at each marker
4. Day 3-4: The data is reviewed by a lab analyst, the calculations are performed, and the report is generated
5. Day 4-5: The report is signed by the laboratory director and released to the customer

Total: 3 to 5 business days for standard testing. Prenatal NIPP testing takes longer (5 to 7 business days) because the lab work is more complex; the lab has to separate fetal DNA from the much larger amount of maternal DNA in the mother's blood sample.