Understanding Your Paternity DNA Test Results - AlphaBiolabs

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Home » Learning Centre » How do I understand my paternity DNA Test Results?

How do I understand my paternity DNA test results?

By Casey Randall, Head of Genetics at AlphaBiolabs

Last reviewed: 17/04/2025

To understand your paternity DNA test results, you need to look at the Combined Paternity Index (CPI) and the Probability of Paternity provided in your test report.

The CPI is a calculation that helps us to arrive at the Probability of Paternity, by comparing the DNA profiles obtained to an untested random individual within the general population. The Probability of Paternity is shown as a percentage, and reflects the statistical probability that the tested man is the biological father.

For example, a probability of 99.99% means that the tested man is more likely to be the biological father than another man chosen at random from the same ethnic group.

Here, we examine 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?

• What are blank DNA results?

• What are positive DNA results?

• 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).

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).

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

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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.

What are blank DNA results?

Blank DNA results – also known as blank DNA test results – mean that no usable DNA was found in the sample, so a DNA profile could not be created.

Although a rare occurrence, this can happen when the DNA sample:

• Doesn’t contain enough cells
• Is inhibited – meaning that the sample contains something (e.g. food or drink residues) that has inhibited the reaction, meaning no profile can be obtained or
• Has degraded

In human identity testing, like paternity tests, this means no comparisons can be made between the DNA profiles of the individuals being tested.

Successful DNA typing relies on a good-quality sample. If the cheek swab isn’t collected properly – for example, if it isn’t rubbed firmly against the inside of the cheek – it might not contain enough DNA for testing.

AlphaBiolabs’ at-home DNA tests have been developed with the customer in mind, with straightforward instructions included in your test kit that enable you to collect your samples quickly, easily and correctly. This helps ensure your DNA samples are usable for testing.

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What are positive DNA results?

In paternity and other human identity testing, a positive DNA result means that the tested individuals share a biological relationship. For example, in a paternity test, a positive result confirms that the tested man is the biological father of the child.

This conclusion is reached through DNA typing, where specific markers in each person’s DNA are analysed and compared. If enough of these markers match between the tested individuals, it provides strong evidence of a biological link.

For an AlphaBiolabs Paternity Test, the test results will show a Combined Paternity Index (CPI) and Probability of Paternity, rather than a positive or negative result.

The CPI is calculated by comparing the DNA profiles of the tested individuals and determining the likelihood that the tested man is the biological father.

This likelihood is then converted into the Probability of Paternity which is shown as a percentage and reflects the statistical probability that the tested man is the biological father.

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

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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Established in August 2004, AlphaBiolabs is an award winning DNA Paternity, Drug and Alcohol Testing Laboratory. We provide testing to the legal profession, corporations and members of the public.

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