DNA Paternity Testing & DNA Paternity Test kits. Home DNA paternity testing

1. About DNA and Chromosomes

DNA (deoxyribonucleic acid) is the genetic blueprint of life. It determines the specific characteristics or traits of each individual. Except for identical twins, every person's DNA is unique. Consequently, a unique genetic profile can be created from a person's DNA. This DNA profile can identify the individual uniquely, and forms the basis of the paternity test.

DNA is a long threadlike molecule found in almost all cells of the human body. An individual's genetic makeup is coded within the DNA in several functional units called genes. The entire DNA content of practically all cells is located in 46 chromosomes. These are complex molecular structures consisting of a long DNA strand and a framework of associated proteins.

The chromosomes are grouped into 23 pairs, of which 22 pairs are homologous, meaning that the paired chromosomes are of the same size and appearance and store information related to the same inherited traits. The 23rd pair comprises the sex chromosomes that are important for sex determination. Each set of 23 chromosomes is inherited from one biological parent. Thus, every individual has two genetic complements in their DNA, one complement inherited from the biological mother and the other from the biological father. This fact forms the foundation on which DNA paternity testing is based, since it allows the biological relationship between a child and an alleged father to be validated or excluded by a comparison of their DNA.

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2. The Paternity Testing Process

In the previous section it was established that by comparing the DNA of a child and an alleged father, it is possible investigate their biological relationship. In practice, only certain regions in the DNA, called microsatellite markers or loci, need to be compared to establish biological paternity. Microsatellite markers are short fragments of DNA in which the same DNA sequence is repeated several times. Since an individual has two genetic complements of DNA, there are actually two copies of each marker present, one inherited from each of the biological parents.

Depending on the laboratory, generally between 10 and 16 microsatellite markers are analysed for determination of paternity. The microsatellite markers used are standard markers recommended by specialised organisations such as the European Network of Forensic Science (ENFSI), the Iberoamerican Working Group on DNA Analysis (GITAD), and Interpol. An important property of these markers is that the number of DNA sequence repetitions (or frequency) is highly variable within the general population but strongly conserved from parent to child. As a result, the frequency of one complement of each of the child's markers matches that of one complement of the biological mother, whereas the frequency of the other complement matches that of one complement of the biological father (see Example 1).

The set of frequencies of the analysed markers constitutes the genetic profile of a tested individual. A standard paternity test determines the genetic profiles of the mother, the child and the alleged father. By comparing the three profiles it is possible to identify the marker in each complementary pair that the child inherited from the biological mother. By elimination, the remaining half of the genetic profile was inherited from the biological father. By comparing this part of the child's genetic profile with the profile of the alleged father, the Paternity Test determines whether the alleged father is the biological father or not. A match between the two profiles indicates that the alleged father is the real biological father (Example 1). On the other hand, if the profile of the alleged father does not match that of the child, he is excluded from paternity (Example 2).

3. Accuracy of a Paternity Test

Since it is so specific, DNA paternity testing is a very powerful form of testing. In a test including samples from the mother, child and alleged father, the probability of paternity is 99.99% or greater when an alleged father's DNA profile matches that of the child for all the genetic markers. On the other hand, an alleged father is 100% excluded from paternity if there is a mismatch between the profiles of the child and alleged father for three or more genetic markers.

When only a child and alleged father are tested (i.e. a motherless test), the information provided by the mother's DNA profile is unavailable. Nonetheless, when there is a perfect match between the DNA profiles of the child and alleged father, the probability of paternity is generally in excess of 99.99%. A mismatch in the two DNA profiles for three or more genetic markers implies with 100% certainty that the alleged father is not the child's biological father.

The accuracy of the test increases with the number of genetic markers included in the DNA profile. Thus, the result of a test that uses a 16-marker profile is likely to be more conclusive than one using a 6- or 13-marker profile. It is therefore important to select a paternity test that uses an adequate number of markers, especially when only the child and alleged father are tested.

Since family members are more likely to have similar genetic profiles, when two alleged fathers are related the probability of paternity may be lower than in standard tests. In such cases it is always best to test both alleged fathers, since the one who is not the biological father can be excluded with certainty. If the two alleged fathers are identical twins it is not possible to identify the biological father, since identical twins have identical DNA profiles.

The reason that it is not possible to determine biological paternity with 100% certainty is that there is always a very small possibility that the profile of the alleged father matches that of the child purely by chance. The likelihood of this happening is generally well below 0.001% (or 1 in 100000) and it depends to a large extent on the ethnic origin of the individuals involved. The certainty of biological paternity generally increases with the number of genetic markers analysed.

Case 1: The figure shows the result for one microsatellite marker from a paternity test that includes samples from the mother (top row), the child (middle row), and the alleged father (bottom row). In this example, the maternal marker that has been passed to the child is number 6. This means that the other marker present for the child (number 7) must have been inherited from the father. In this case the alleged father matches the child, since one of his markers is indeed number 7. This procedure is repeated for all microsatellite markers used in the test.

Case 2: The figure shows the result for one microsatellite marker from a paternity test that includes samples from the mother (top row), the child (middle row), and the alleged father (bottom row). In this case the maternal markers are 29 and 30. This implies that the child has inherited 29 from the mother and the 31.2 complement must have been inherited from the biological father. Since the alleged father does not possess this marker it is unlikely that he is the biological father. In practice this mismatch between the child and alleged father's DNA must be present in at least three markers to exclude paternity with certainty.