Why We DNA Test The Mother

Casey Randall AlphaBiolabs

By Casey Randall, Head of Genetics at AlphaBiolabs

Last reviewed: 22/07/2017

Why we ask for a DNA sample from the Mother

AlphaBiolabs currently perform around 2500 DNA paternity tests per year from a wide range of people each with their own reasons for needing a test.

With this high volume of tests being performed there are often questions raised by both the instructing parties and the participants of the test. One of the commonest is “I know I am the mum so why are you testing me?”

It is not uncommon for us to test to determine maternity but in the majority of cases it is the paternity that is in doubt so why do labs require the mother’s sample?

The Human Genetics Commission and the Department of Health

The Human Genetics Commission and the Department of Health both recommend that the mother should be at least aware of the test if not directly involved and it is viewed that motherless testing could be “harmful to the child, as well as the family unit as a whole”.

All reputable labs would strongly recommend the inclusion of the mother’s sample for this reason alone; however there are also scientific reasons to include the mother in a test.

Half of a child’s DNA comes from the mother and the other half from the father.

The Probability of Paternity

A DNA test determines whether or not a tested man could have passed on his DNA to the child. When the mother is included it is possible to identify which of the child’s DNA comes from her, leaving the paternal DNA to compare against the alleged father. DNA test reports usually include two values; the Combined Paternity Index (CPI) which provides an odds ratio of how much more likely it is that the tested man is the father rather than an untested man and the Probability of Paternity which reflects the probability that the tested hypothesis is correct.

These values can be calculated with or without the inclusion the mother’s sample but provides much greater certainty when the mother is tested. For example, a test that we recently completed with the mother gave a CPI of 56,836,601 and a Probability of Paternity of 99.99999% indicating it is 56,836,601 times more likely that the tested man was the father than another man; removing the mother from this test would have reduced this to around 10,000, more than 5,500 times lower.

In some cases this reduction could be even greater and reduce the CPI and probability so much that the test is no longer conclusive or as definite as it could have been, in which case it may be that the mother’s sample would be required anyway, leading to more paperwork and delays to the result.

In some circumstances the mother’s sample cannot be provided or the delay to a case because of an evasive sample donor can mean that motherless testing is unavoidable, but when the cost of a DNA test includes the mother’s sample it really is in the best interest of the test results to include her in the process.

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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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