Most people know that analyzing and comparing DNA is at the heart of any paternity test. But what exactly is the science behind a DNA paternity test ? How does it work? Here’s a quick overview.

What is DNA?

DNA is everywhere.  Your body contains as many as 100 trillion cells all serving important functions, from protecting your organs and carrying oxygen, to creating life through reproduction. Despite the incredible diversity of cells, each cell (except for red blood cells) shares a single important trait: a nucleus. The nucleus holds genetic information that is identical from cell to cell, bundled into molecules called Deoxyribonucleic Acid, or DNA for short.

Every cell contains the same genetic information, and every cell on your body is a potential source of DNA; skin cells, hair roots, and cells carried within saliva provide just a few examples.

Collecting DNA Samples for Paternity Testing

Paternity testing requires a clean sample (a sample free of contaminants and foreign DNA) with a good amount of the participants’ DNA. While HomeDNA Paternity can extract DNA from almost any specimen type, our standard DNA paternity test uses easy and non-invasive cheek swabs. HomeDNA Paternity customers rub the swab against the insides of their cheeks for about thirty seconds.  The swab’s special mesh tip (it’s not cotton like a Q-Tip) grabs loose cells for DNA testing and typically provides more than enough DNA for a single use, such as a paternity test.

DNA Paternity Test Science: Extracting the DNA

Genetic scientists and technicians use many methods to extract DNA from cells. All methods use three basic steps:

Using a special chemical process, we break open the collected cells. This separates the DNA from the nucleus and leaves the scientist with a liquid that contains DNA along with other cell parts not needed for testing, such as proteins and lipids. We then separate the DNA from the other cell parts using sophisticated robotics.

The extraction robot uses more chemicals to transfer the DNA mixture to tiny silica-based nano-beads (one billionth of a meter in size); the DNA sticks to the beads while the other cell parts are washed away.

The final step, the elution step, removes the DNA from the beads. The robot collects the pure DNA, which is now ready for the next step in the paternity testing process.

DNA Paternity Test Science: PCR – Amplifying (Copying) the DNA

The DNA scientist puts the extracted DNA into a special solution containing primers.  Like toner in a biological copy machine, primers find and make copies of the DNA sample-just those specific regions that we need for a paternity test.

The copy process begins by separating the double-stranded DNA – simply by turning up the heat. As the solution cools, the primers bind to single-stranded DNA, making two copies of the original.  We repeat this process (heating and cooling the DNA and the primers) 28 times, making millions of copies of tiny DNA fragments that can now be detected and viewed by a special machine called a genetic analyzer.

Scientists refer to this ‘biological copy machine’ process as Polymerase Chain Reaction, or PCR.

DNA Paternity Test Science: Measuring DNA for Paternity Testing

The complete PCR process makes copies of 16-18 Genetic Systems (sometimes called markers or loci) to make one DNA Profile: 15-17 markers useful for paternity and one (1) gender marker (used for test-participant verification).  Each individual person has different sizes or lengths of DNA fragments at each Genetic System. Special software measures the different sizes of the DNA sections, represented by two numbers (alleles) at each Genetic System on your paternity test report. We then use this information to answer your paternity question.

Paternity Test Reporting

A child’s DNA Profile is always a combination of half the father’s markers and half the mother’s markers. If the tested (possible) father does not share matching markers with the child, then the tested man is excluded as the biological father (he is not the father). If the DNA Profiles do match, the father is not excluded (he is the father) and the probability of paternity is reported (typically greater than 99.99%).

Top 3 Questions about Paternity Test Results

1. Q: I got 0% probability of paternity, but the child and I still have some genetic markers that match. How does that happen? 

A:  It’s actually not unusual at all for there to be several matches between the tested parties, even though the alleged father and child are not related. This happens because, as human beings, we’re all bound to share some of the same genetic material. But, with the exception of a mismatch caused by a mutation, in order to be considered the biological father, the man tested and the child tested must have a match at every marker tested.

2. Q: At one genetic location, I have two numbers, but the child only has one. What does that mean? 

A: Whenever there are two of the same number at one genetic location, the report shows it as just one. For example, if one of the child’s genes is 11,11 then it means they got one 11 from mom and one 11 from dad. But it shows on the report as just a simple 11.

3. Q: I tested without the mother. If she participates too, does it change the results? 

  A: Most likely not. But adding the mother can strengthen an inclusion (you are considered the father). For example, adding the mother could make the difference between a 99.3% probability of paternity and a 99.9% probability of paternity. Both of these are considered conclusive results.

For more information on how to understand your results see, What Do My Results Mean?

Other Types of DNA Tests

Paternity testing is a popular use for this technology, but as the science has gotten more and more sophisticated, other “lifestyle” tests have become all the rage with many consumers. There are DNA tests for ancestry, skin care, weight loss and nutrition, and even tests for dogs! It truly is a brave new world and the possibilities are endless.

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