Parentage testing is the examination and comparison of an alleged parent’s and a child’s genetic profiles to determine whether the individuals are biologically related as parent and child. There are two types of parentage tests: paternity tests and maternity tests.
Parentage testing is based on the fact that a child inherits half of his or her DNA from each biological parent. At conception, the biological mother contributes one-half of her genetic profile to the zygote through the egg cell, while the biological father contributes one-half of his genetic profile to the zygote through the sperm cell. Because DNA is inherited in this way, a child’s genetic profile can be compared to an alleged parent’s genetic profile to determine whether the two are biologically related. The profile of a biologically related child will show predictable patterns of the genetic inheritance.
The following is a primer on how parentage testing works.
To begin a parentage test, DNA is collected from the tested parties. The most common method of obtaining DNA samples is the painless buccal swabbing method, but DNA can also be obtained from blood or tissue samples and, many times, from ordinary items such as chewing gum, cigarette butts, and clothing stained with blood or semen.
In a paternity test, DNA samples must be collected from the alleged father and the child; the child’s mother may also be collected if she is available. In a maternity test, samples must be collected from the alleged mother and the child; if available, the child’s father may be tested as well.
Once a sample is collected from a tested party, the DNA is extracted from it, purified, and replicated. The replicated DNA then undergoes capillary electrophoresis, which produces a DNA profile.
A person’s DNA profile is made up of a number of genetic markers, which are specific sequences of DNA at particular locations that scientists have determined to be useful as identifying marks for individuals. Every person has two copies of these markers; one set is inherited from the mother, and the other set is inherited from the father.
A DNA profile reveals the allele sizes of the different markers, and in a parentage test, these are what the DNA analyst compares to determine whether the alleged parent and child are biologically related. If the tested parent and child are biologically related, their profiles will have at least a 50% similarity.
When the allele sizes of the DNA markers at a specific locus match between an alleged parent and a child, the marker is given a Paternity Index (or Maternity Index) value. This value depends on the frequency with which the marker appears in the population. When the allele sizes of the DNA markers at a specific locus do not match between an alleged parent and a child, the marker is given a Paternity/Maternity Index of 0.
After all the markers have been compared, the Paternity/Maternity Indexes are multiplied. This produces the Combined Paternity/Maternity Index, which is converted into a Probability of Paternity/Maternity percentage. The Probability of Paternity/Maternity percentage states the probability that the tested parent is the biological parent of the child. For an inclusion result, the Probability of Paternity/Maternity is 99.9% or higher.
As an industry standard, if an alleged parent’s profile shows three or more DNA markers that do not match the child’s markers, then that person is excluded as the biological parent of the child. (The Probability of Paternity/Maternity is 0%.)
In addition to parentage tests, companies in the parentage testing industry usually perform other family relationship tests that operate under the same basic principles as paternity and maternity tests. Examples of these other family relationship tests include grandparentage tests and siblingship studies.