How do I understand my paternity DNA test results?

Casey Randall AlphaBiolabs

By Casey Randall, Head of Genetics at AlphaBiolabs

Last reviewed: 05/10/2023

Common questions about understanding your paternity test results.
Table of contents
  • What are DNA STR markers?
  • Which DNA STR markers are examined?
  • How does the test work?
  • What is the probability of paternity?
  • What is the combined paternity index?
  • Could the result be incorrect?

What are DNA STR markers?

Short tandem repeat (STR) markers are specific locations on a chromosome made up of sequences of repeated DNA. The STR markers that AlphaBiolabs examines are highly variable in length between individuals. Each individual has two copies of each STR marker, known as alleles: one is inherited from the father and the other from the mother.

Which DNA STR markers are examined?

AlphaBiolabs tests up to 45 STR markers including two sex markers as standard. These are:

  • DNA markers: D3S1358, vWA, D1S1358, D16S539, CSF1PO, TPOX, D8S1179, D21S11, D18S51, Penta E, D2S441, D19S433, TH01, FGA, D22S1045, D5S818, D13S317, D7S820, D6S1043, D10S1248, D1S1656, D12S391, D2S1338, D7S1517, D3S1744, D2S1360, D6S474, D4S2366, D8S1132, D5S2500, D21S2055, D10S2325, SE33 and Penta D
  • Two sex markers: Amelogenin and Yindel (Y chromosome specific)

The Amelogenin gene can be used to determine the sex of the individual being tested. There is only one copy of the Amelogenin gene on each of the sex chromosomes, which allows us to determine if the sample has come from a male (XY) or female (XX).

How does the test work?

The two alleles observed at each STR marker are compared between the tested individuals. A full match of these alleles between individuals provides evidence of a relationship between them. Each allele is represented by two numbers. For example, if a father displays numbers 13 and 17.3 for one specific genetic marker, the child will need to display either the 13 or the 17.3 on the same genetic marker to show it has been inherited from the father. There will need to be a match with all the STR markers tested for an inclusion of paternity (A).

Paternity test case example

STR marker Mother Child Possible father
D3S1358 11 17 11 14 8 14
vWA 16 16 15 16 15 15
D16S539 14 15 15 17.3 13 17.3
CSF1PO 10 10 10 10 10 10
TPOX 13 14 14 14 14 16
AMEL X X X X X Y

In example A, you can see that the child’s DNA footprint is made up from half the mother and half the father. In example B below, you can see that the potential father does not share a marker with the child and is therefore excluded from paternity (i.e., he cannot be the father).

Paternity test case example

STR marker Mother Child Possible father
D3S1358 11 17 11 14 8 14
vWA 16 16 17 16 15 15
D16S539 14 15 15 17.3 13 17.3
CSF1PO 10 10 11 10 10 10
TPOX 13 14 15 14 14 16
AMEL X X X X X Y

What is the probability of paternity?

A Laboratory testing relies on the statistical probability of the alleged father being the child’s biological father and not any other man from the same ethnic group who may share a similar DNA profile by random chance.

What is the combined paternity index?

The probability of relationship is calculated by comparing the DNA profiles obtained to an untested random individual within the general population. When the probability of paternity is 99.99% for example, this means that the tested man is 99.99% more likely to be the biological father than another man chosen at random from the same ethnic group. The combined paternity index is a calculation that helps us arrive at the probability of paternity.

Could the result be incorrect?

  • AlphaBiolabs tests all exclusion results twice so you can be confident of getting an accurate result.
  • Most illnesses will not affect the DNA result.
  • All tests are performed assuming that a close male relative of the alleged father is not the potential father. If this is not the case, it is recommended that any such relative is also tested as the result provided may be invalid. This information should be provided at the time of testing so that it can be taken into consideration when analysing your results.

Further Frequently Asked Questions and lots more information can be found on the Learning Centre pages of our website: www.alphabiolabs.co.uk

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Casey Randall AlphaBiolabs

Casey Randall

Head of Genetics at AlphaBiolabs

Casey joined the AlphaBiolabs team in 2012 and heads up both the DNA and Covid-19 testing teams.

An expert in DNA analysis and a member of the International Society for Forensic Genetics (ISFG), Casey holds an MSc with Distinction in DNA Profiling and a First-Class BSc with Honours in Forensic Science.

Casey is responsible for maintaining the highest quality testing standards, as well as looking for ways to further enhance the service that AlphaBiolabs provides and exploring new and innovative techniques in DNA analysis.

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