paternity-test

Paternity tests – How do they work and how accurate are they?

Introduction

“Who is the father of the child” and “who is my biological father” are questions that arise often, not only in plots for movies and novels, but in courts or simply for peace of mind. Paternity tests offer answers to these questions with scientific evidence. 

With current technology, DNA paternity tests results have an accuracy of over 99.99%. Further, results are often available in less than a week. But, how exactly do these tests work and where can you get one?

DNA inheritance

Each one of us is born with a unique genetic blueprint. DNA is this unique “fingerprint” that makes up a person’s genetic material. This genetic material is derived from the DNA of both parents. Half the DNA is inherited from the biological mother and the other half from the biological father. This is the premise on which paternity testing is built. First, samples of DNA from the child and the potential father are collected. By analyzing the results, the probability of parentage is determined.

History of paternity testing

Tests for paternity identification and family relationship linking have been used for more than 100 years. In the early twentieth century, blood typing was the most common method. 

This method takes advantage of the heritability of blood types, which are A, B, AB, and O. Because blood type is encoded in DNA, a child inherits one allele from each parent. So, assessing a child’s blood type can give insights about the potential blood types of the parents. 

For example, a person with blood type O must have two O alleles, one inherited from each parent. But it is not always this straight-forward to determine. An individual with type A blood can have either one A allele and one O allele, or two A alleles. See Figure 1 for a chart with a child’s blood type possibilities.

Figure 1. Inheritance of blood groups. A child’s blood group is determined by the alleles received from the mother and the father. Each biological parent donates one of their two ABO alleles to their child. Blood type A constitutes at least one copy of the A allele, but they could have two copies. The genotype is either AA or AO. Similarly, someone with blood type B could have a genotype of either BB or BO.
Figure 1. Inheritance of blood groups. A child’s blood group is determined by the alleles received from the mother and the father. Each biological parent donates one of their two ABO alleles to their child. Blood type A constitutes at least one copy of the A allele, but they could have two copies. The genotype is either AA or AO. Similarly, someone with blood type B could have a genotype of either BB or BO.

Unfortunately, this type of test is limited in the information it can provide regarding a child’s parents. It cannot confirm a biological relationship like fatherhood. Instead, it can only act to exclude a potential parent based on possible blood type.

With advances in DNA testing technology, paternity testing has shifted towards accurate confirmation of paternity rather than simple exclusion. 

The science behind DNA-based paternity testing

The DNA fingerprinting technology first described in 1985 has now become a powerful tool for paternity and maternity testing. It first became available for in 1988 and has since evolved. The current gold standard employs a molecular biology technique that enables exponential amplification of fragments of DNA. 

The procedures are simple, only requiring a small sample of bodily fluids/tissue from the child and the potential biological father. This sample is typically gathered from a blood draw or by swabbing the inside of the cheek.

Paternity testing based on DNA has become the most accepted method for proof of biological relationship. Since half a person’s DNA is inherited from the mother and the other half from the father, comparison of DNA sequences can identify parentage. 

Sibling DNA tests can also be performed in cases of a deceased or missing parent. These sibling tests can be used to verify if two or more individuals share a common parent. The pattern of inheritance of genetic markers between siblings are different from that between a parent and child. While identical twins share 100% of the same DNA, full siblings only share ~ 50% of the DNA. Half-siblings only share 25% of the DNA, on average.

Methods for paternity testing 

DNA paternity testing involves collection of a DNA sample from a child and the potential father. The DNA sample may be collected using multiple methods. The most common methods are collecting a blood sample or a swab from the inside of the cheek. The DNA is fragmented using one of multiple approaches and the fragments are run on a gel that separates them by size creating a distinct banding pattern (Figure 2). The patterns of these bands provide information about the biological relationships, as described below. 

DNA profiling in paternity testing. Children inherit half their DNA from each parent and thus possess a combination of both. DNA fragments from the child should also be present in the fragments from the mother (in grey) and the father (in black). Here, Father 2 is more likely to be the biological father.
Figure 2. DNA profiling in paternity testing. Children inherit half their DNA from each parent and thus possess a combination of both. DNA fragments from the child should also be present in the fragments from the mother (in grey) and the father (in black). Here, Father 2 is more likely to be the biological father.

There are two methods for generating DNA fragments. The first is restriction fragment length polymorphism (RFLP) that utilizes the variation in DNA sequences recognized by restriction enzymes to cut DNA in pieces. The second is polymerase chain reaction (PCR) that takes advantage of rapid amplification of DNA, either at unique sequences or short tandem repeats (STR).

RFLP

Restriction Fragment Length Polymorphism is a method that takes advantage of unique DNA patterns in specific regions across an individual’s genome. A large family of proteins known as “restriction enzymes” can identify and cut DNA at these unique sites. As a result, fragments of variable sizes are produced depending on a person’s DNA profile. 

By looking for similar patterns in the digested fragments between the individual and the potential father, paternity is determined (Figure 2). Since each restriction enzyme has a unique sequence it recognizes, small differences in the DNA will result in cutting at different sites. By comparing the pattern of the DNA fragments cut by the restriction enzyme, paternity can be confirmed. Although this technique has been used for paternity testing and forensics, it can be a very slow process. 

PCR

Polymerase Chain Reaction is a method where fragments of an individual’s DNA are copied billions of times in a “chain reaction”. This allows large amounts of DNA to be created from only a small amount of template DNA. These fragments are then labeled with fluorescent tags that allow for visualization of the fragments on a gel. If roughly half of the fragments analyzed to match the child and the potential father, paternity is confirmed with an accuracy of 99.99% (Figure 2).

PCR has become the gold standard for paternity testing, and boasts a higher accuracy than RFLP. Additionally, the PCR-based method requires a relatively smaller sample of DNA than the RFLP method. 

DNA sampling for paternity testing before birth

While paternity testing can be easily carried out by collecting blood or cheek swabs from individuals, it is challenging if the test needs to be carried out before birth.

Questions often arise before birth and early testing is desirable, especially if the woman considers terminating the pregnancy.

There are three ways to collect a DNA sample from an unborn child: Chorionic Villus Sampling (CVS), amniocentesis, and the Non-Invasive Prenatal Paternity (NIPP) test.

Chorionic Villus Sampling (CVS) 

This DNA sample collection method samples chorionic villi which are little finger-like projections on the placenta in the uterus. The genetic makeup of the chorionic villi and the fetus are identical. To obtain the chorionic villi tissue sample, a catheter can be inserted through the cervix into the placenta (transcervical method). A needle can also be inserted through the abdomen and uterus into the placenta (transabdominal method). 

This procedure is guided by ultrasound and due to high risk of complications, a doctor’s consent is needed. This testing is usually done between 10-13 weeks of pregnancy.

Amniocentesis

This DNA collection method uses amniotic fluid that surrounds the fetus. The amniotic fluid contains cells that are shed by the fetus and contain genetic material that can be analyzed. To obtain a sample of the fluid, a long, thin, hollow needle is pierced through the skin, into the uterus, and into the amniotic sac, to draw a sample. 

Although invasive and only to be carried out with a doctor’s consent, risks of miscarriage is considered less than 1%. This test can be performed in the second trimester, between the 14-20 weeks of pregnancy. 

Non-Invasive Prenatal Paternity (NIPP)

This is the most accurate prenatal paternity test that is non-invasive. The process is based on analysis of free-floating DNA from the fetus found in the mother’s bloodstream. This test requires only a simple blood sample collection from the mother, and a blood draw or cheek swab from the alleged father. 

This test can be performed as early as the 8th week of pregnancy. The accuracy of the test is 99.9% and recommended by the American Pregnancy Association. If the woman was pregnant recently, there may be fetal cells in the systemic circulation that may invalidate the test results.

Due to the availability of a safe and reliable non-invasive test, the two invasive techniques are instead only reserved for diagnosing genetic diseases of the fetus.

Legal issues surrounding paternity testing

Both legal and non-legal tests are available. Non-legal testing includes take-home DNA tests that provide the same information as legal paternity tests. The difference is that information cannot be used in a court of law and is not considered court-admissible. Only court-ordered tests may be used in court cases and for legal purposes.

The laws and regulations surrounding paternity testing vary widely across the world. It is important to consider the rules in your country before proceeding with testing.

In the United States, paternity testing is fully legal and does not require the consent of the mother. There are state-to-state variations in the legal implications of a test, especially for child support or custody. For example, New York has different testing regulations than other states. The state requires authorization from a physician or a lawyer for the test. As a result, testing at home is also not allowed for NY residents.

In Canada, testing is more regulated with certified labs only having the approval to perform paternity tests.

The United Kingdom had no restrictions on testing till 2006. According to the Human

Tissue Act, any testing can only be performed with the consent of the individual.

Paternity testing is legal in China. It is legally required by the government to investigate any child born outside the one-child policy before a birth certificate is issued. Peace-of-mind tests are also popular in China for confirming parentage. 

France has made it illegal to perform any tests on the human body including genetic testing, unless ordered by court or to identify a deceased individual. DNA paternity testing is punishable by a €15,000 fine and up to a year in prison.

Companies offering DNA paternity testing services

LabCorp offers both legal tests and collections kits for at-home tests to establish paternity. They have been providing these services since 1981 with highly accurate results, excluding, on average, 99.99% of non-fathers. Their legal test starts at $525 and take-home tests start at $210.

Easy DNA offers various testing services. This includes legal tests, home paternity tests, immigration DNA tests, sibling DNA tests, Non-Invasive Prenatal tests, and forensic tests. Their home testing services start from $199, and have an accuracy of 99.9%, without testing the mother. With the mother’s DNA sample, the accuracy of results up to 99.99%.

Amazon sells the STK Paternity Test, a take-home paternity testing kit for the parent and child. The DNA tests are carried out in an accredited laboratory. This is available for $99.99 with results available in less than a week. 

Testing kits are also available at many drugstores. One such test is the HomeDNA paternity Test (formerly IDENTIGENE), sold by HomeDNA. The kit is available for $14.99 with an additional fee of $139 for lab services. Results are returned in two business days, with one-day and same-day options available for an additional fee. You can read more about HomeDNA testing in our review.

You may also be interested in learning about DNA testing during pregnancy!

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