DNA IN PARENTAGE TESTING
UPDATED FOR THE WEB EDITION, April 2000
In the United States, establishing paternity has become a major industry. Government efforts to recover child support funds bring in over five billion dollars a year at a cost of one-and-a-half billion dollars. State and local child support agencies, working under Title IVD of the federal Social Security Act, Aid to Families with Dependent Children, administer these programs for the benefit of unsupported children. This statute requires that when a woman obtains public assistance for her children, she MUST name the possible fathers. These programs generate the bulk of more than 220,000 paternity tests performed annually. In a country where 30% of births are to single mothers, and 15% of fathers of record are not the biological father, the large number of tests is not surprising.
Parentage testing cases are numerically the largest user of DNA testing. More than 200,000 of the paternity tests performed in the United States every year use DNA testing exclusively or as a supplement to conventional blood testing. HLA (Human Leukocyte Antigen) and other classical blood typing tests often can resolve a routine paternity case without resorting to DNA testing. The HLA blood types are the types that must be matched before an organ transplant, such as bone marrow, kidney or heart, can succeed. Limitations of conventional blood testing are that the blood must be fresh when tested, and only a limited amount of genetic information is obtainable. Also non-blood cell types, such as those found in semen, do not have an HLA and most other common blood types, rendering this type of testing much less useful in forensic cases. The advantage of DNA testing is that many more variable genes, up to thirty or more, are available. In effect, our ability to do paternity testing is no longer limited by the amount of genetic information that we are able to obtain.
Most paternity testing is done for financial reasons, i.e., to establish legal responsibility and provide for child support. Even more important are the emotional and social issues. When testing can demonstrate conclusively to a man that he is the father of a child, then he is more likely to provide not only financial support, but emotional support as well. He may bond with the child and take an active part in its life. The importance of establishing paternity early is nowhere more clearly shown than in the testing of adult children, some of whom are in middle age. "Who is my father?" is a question that may have haunted them their entire lives.
It wasn't very long ago that obtaining a paternity test was an uncertain, expensive and inconvenient process. It was hard to find a place to have the blood drawn to initiate the testing and redraws were common as the blood had to reach the laboratory in one or two days. A doctor's or a lawyer's involvement might have been required, and, even after the effort and expense, the results often came back with a considerable degree of uncertainty. Also, there were no inspection and certification programs available to parentage testing laboratories which provide an assurance of quality.
DNA testing has made the process convenient and the results conclusive. The testing is still expensive and takes a week or two to complete, but anyone can order it. In all but the rarest of instances, the DNA test results provide a level of certainty so high that paternity will, for all practical purposes, be proven or disproved.
One of the greatest benefits DNA has brought to parentage testing is the ability to solve many more types of cases. In the usual case, tests include all three parties, i.e., the mother, child and alleged father. With DNA, it is not necessary to have the mother's sample in order to provide a conclusive test result. It is possible to distinguish between two brothers provided they are not identical twins. In many cases, it is possible to perform the DNA tests when the alleged father is deceased or otherwise unavailable. It also is possible, using DNA, to perform testing before or at the time of birth. Tests also can be performed on some very unusual samples such as envelope flaps, cigarette butts, and very old blood stains.
What Happens at the Sample Collection
Throughout this country, and the entire world, is a network of individuals licensed and experienced in the legal sample collections that are necessary in paternity testing. These collections differ from medical collections by the extent to which they are documented. Clients who are being tested for paternity will be asked to show a picture ID, fill out and sign release forms and identification labels, and a photo or thumbprint will be taken. In this way the chain of custody (COC) that must be maintained throughout the transport and testing can be assured. In order for the COC to have maximum integrity the tested parties should not be involved in the shipping or handling of the transport kit. It should be sent directly to the collection site or personal physician, and include pre-addressed return airbills with instructions.
Only a small volume of blood, a teaspoon or less, will be taken in a draw. Children, even babies, usually tolerate it quite well. The blood samples from babies also are drawn from the arm because it is less painful and much quicker than a heel or finger stick. Unlike former practices, blood now can be drawn any day of the week, and all parties need not be drawn at the same time or place.
Buccal or Cheek Swabs and Home DNA Paternity Tests
More and more collections are performed using a "buccal swab," a Q-tip that has been rubbed on the inside of the cheek. This type of collection has made the "home paternity DNA test" possible. The home test is the ultimate in convenience. Unfortunately, it has minimal legal standing because it is generally impossible to independantly verify the identity of the individuals who have been tested. There is no chain of custody. Another type of home DNA test overcomes these obstacles because the collection is performed in home by a medical professional who comes to the home. This level of service is not available everywhere but is becoming more available all the time.
PARENTAGE TESTING APPLICATIONS
Prenatal and Newborn Paternity Testing
Formerly, it was impossible to test babies until they were six months old. Testing blood from the umbilical cord of a newborn now is one of the most convenient and effective ways to obtain a paternity test. At the time of birth labor and delivery staff will take blood from the umbilical cord after the baby is born, collect from the mother and ship the samples to the laboratory. Arrangements must be made in advance and if the alleged father has been drawn prior to the birth, testing can start immediately.
One of the fastest growing areas DNA paternity tests is prenatal testing. While it has been possible for many years to perform a prenatal test for paternity, it has not been routinely done because of the small risk to the fetus and the long turn around time (six to eight weeks.) That is changing as the amniocentesis and chorionic villus sampling procedures become more accepted and routine. These are the procedures for obtaining cells from the unborn baby which, in the past, have been limited to women over thirty-five or for families with a history of genetic disease. With the latest in DNA testing methods results can be obtained in a matter of days.
Deceased Alleged Father
This type of DNA testing often is needed to obtain Social Security or insurance benefits, to settle an inheritance dispute or to establish standing in a wrongful death action. There are two basic approaches to solving these cases. The first is to obtain a postmortem sample of the father's DNA. The second is to reconstruct his genotype based on surviving family members. Samples from the deceased may be obtained from the medical examiner, coroner or other pathologist who may have performed an autopsy, an associated toxicology or medical lab, the funeral home before or after embalming, or even following burial for up to several years.
Kinship Analysis - Missing Alleged Father
If samples from the deceased are not readily available then the alleged father's genotype can be reconstructed by testing close relatives. The most straightforward of these tests is a grandpaternity test in which the alleged father's parents are tested. If one or both of his parents are missing, then samples from his siblings or other children can be used and extended typing performed. Any combination of three or four of these close relatives will result in a highly conclusive test. When fewer relatives are available for testing, the ability to provide a conclusive result decreases. Relatives farther removed from the alleged father such as cousins are not related closely enough to provide useful information. One caution to this type of testing is that if the relationships are misrepresented to the laboratory, a false negative may result.
The most difficult kinship analysis is to determine if two individuals are full or half siblings when those individuals are the only ones available for DNA testing. If the number of tested siblings increases or other relatives become available, the degree of certainty increases dramatically. It also is easier to determine if two individuals are full siblings or completely unrelated. Advanced genetic methods and specialized computer programs are required to reliably solve these cases. It may be necessary to test twelve or fifteen or more genes in order to make a determination.
The most common and most disturbing criminal paternity cases are those involving the sexual abuse of a child. Laboratory testing is not possible in many of these cases because the abuser is a relative or close friend of the family, and the crime comes to light after the physical evidence is gone. Sometimes these unlawful sexual contacts do result in a pregnancy, especially if they continue over an extended period of time. In this instance, the DNA testing is usually routine and can be performed on the child or an abortus that is older than six weeks. DNA testing may be especially useful if there are closely related suspects, such as a father and son or two brothers. Criminal cases also may require the application of non-routine statistical and reporting techniques depending on the requirements of the court.
Cases where the victim of sexual abuse is underage rarely go to court. The offender usually will receive a lower prison term if he pleads guilty. Conviction in the face of DNA evidence is almost inevitable. In cases where the victim is an adult and consent is a defense, the defendant may change his story when he realizes that the DNA testing results are almost impossible to refute. The jury is usually unaware of this change of story and so often will vote to acquit the defendant.
DNA parentage testing has been questioned by the courts in only a handful of cases. Typically, less than 1 or 2 percent of civil cases result in a case contested in trial. The opportunity to have testing repeated by the same or another laboratory usually can settle most disputes to the satisfaction of all tested parties. In criminal cases, when the testing is scrutinized extensively in court, it is almost universally accepted.
Genetic Reconstruction or Reverse Paternity
These techniques often are used in homicide investigations. Suppose that there is a tissue sample from an unidentified deceased individual, or a stain from a crime scene and investigators have an idea of who may be the sample's source. If the decedent's parents or children are available, it will be possible to compare the related survivors' DNA profiles with that of the deceased to conclusively determine whom the remains came from.
Paternity testing is possible because everyone has two copies of their genetic information, half from each of the parents. If a child has a gene that didn't come from the mother, then it must have come from that child's biological father. If the alleged father does not have that gene, the obligate gene as it is called, then he cannot possibly be the father. He is excluded from the group of men who could be the father of the child. There is one exception. If a mutation has occurred, which means the gene changed when it was passed from the father to the child. These mutations rarely occur, but they occur often enough that statistical methods are available for dealing with them and two DNA exclusions are required before reporting out a DNA paternity test result.
The other possible outcome is that the alleged father has the obligate gene and so is included in the group of men who could be the father of the child. In effect, what paternity testing does is examine a series of genes until the group of men who could be the father of the child is narrowed down to the point that we can, for all practical purposes, be certain that he is the father. In DNA testing, this can be accomplished by testing as few as three highly variable genes.
Two different ways are used to report the two possible testing outcomes. An exclusion is reported as a simple statement that the alleged father is excluded from the group of men who could be the father. In the case of an inclusion, a statistical analysis is performed based on how common or rare are the obligate genes. These are the genes that the alleged father could have contributed to the child and that must have come from the child's biological father. The inclusionary results are expressed in the following two ways:
The Paternity Index
The first is the paternity index (PI), which is a simple odds ratio. Because of the increased accuracy possible with DNA testing, the generally accepted minimum standard for an inclusionary result has risen to a PI of 100. This means that the alleged father has a 99 to 1 better chance of being the father than a random man. It does not mean that the test is only 99 percent accurate. A more realistic way to look at this result is to consider that the alleged father has a PI of 100 and that the genetic profile he must donate is found in approximately 1 in 100 randomly chosen men. If he has been falsely accused, then he has the required 1 in 100 profile. The probability of these two events occurring together is more realistically thought of as 1 in 10,000.
DNA test results typically produce odds ratios of hundreds or thousands to one. As a practical matter, once a PI of 100 is reached, the probability of overturning that result with further testing becomes extremely small. In practice, the test results are reported out according to the race of the alleged father. While the results can vary slightly from race to race, as a practical matter, when modern DNA testing methods are used, the race of the father doesn't make a significant difference.
Probability of Paternity
In the second way, the probability of paternity is calculated simply by converting the PI to a percentage. However, it can be the most confusing because the equation that is used for the calculation contains a term called the "prior probability." This term takes into account the non-genetic factors that would have a bearing on paternity. In the laboratory a neutral value of 0.5 is universally used. It favors neither the alleged father nor the mother.
The paternity index and probability of paternity represent a comparison of the alleged father against a random man in the population. If there is a close relative who also could be the father he also should be tested. If he is unavailable, then the laboratory can take that fact into account when doing the testing and calculations.
Choosing a Laboratory
Because paternity laboratory inspection is not mandatory, there is a wide range of quality among the laboratories. On the fringe are a number of small laboratories which do a low volume of testing and do not subscribe to the established quality control assurance programs. In evaluating a parentage testing laboratory, one should ask if it is accredited in DNA testing by the American Association of Blood Banks. Labs may be accredited in the older types of testing, such as HLA and red blood cell testing, but not in DNA testing.
When contacting a laboratory, if it is not possible to talk with an understanding, helpful and knowledgeable client services representative, it might be advisable to look elsewhere. A lab may do fine work, but if there is not someone available to clearly explain the process and results, it can be frustrating. Another factor which can help in laboratory selection is whether or not the laboratory has provided testing and testimony in criminal cases, which require the most advanced expertise.