Paternity Test - Comparative DNA Testing |
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In forensic pathology, DNA fingerprinting is used to single out one suspect from many possible subjects and to provide odds that the DNA at a crime scene matches the suspect's DNA. In paternity testing, by contrast, DNA fingerprinting is used to identify a particular individual and relate that person to another individual.
In most cases, the reason for paternity testing is to determine whether a man has fathered a particular child. Traditionally the blood type of the man was compared with that of the child. The results might exclude the man as the child's father but they could not definitely point to him as the father. For example, if the man had blood type AB and the child had blood type O, the man could not possibly be the father because he would contribute the gene for A or B blood antigens to any child he fathered; and neither gene is present in this child. By contrast, if his blood type was O, then he could possibly be the father - but many other men could be the father as well.
The advantage of DNA fingerprinting in this situation is that only two types of DNA must be compared: the man's and the child's. Either the DNAs will match, in which case he is declared the father, or they will not match, in which case he is excluded from paternity. In a match, half the child's genetic pattern will be identical to half of the father's pattern because half the genes have come from him. The gene patterns are therefore compared as in DNA fingerprinting to see whether the father and child have the same number of base sequence repeats. DNA fingerprinting of this type yields such persuasive evidence that trials are often considered unnecessary when the test results are presented. An estimated 120,000 paternity suits are heard annually in the United States.
An unusual paternity case surfaced in 1997. A woman in Spain gave birth to a pair of fraternal (nonidentical) twins, and the husband, suspecting they were the result of an affair, demanded paternity test to see whether he was the father. To his surprise, DNA analysis showed that he was the father of one child and another man was the father of the second. After questioning, the woman admitted she was having an affair. Apparently she had produced two egg cells that month, and each was fertilized by a different man's sperm.
During the late 1980s, research evidence indicated that DNA could be extracted from bones at archaeological sites. This finding is critical not only to the hunt for ancient DNA but also to modern forensics because it means that genetic identification can be made of the recently deceased. During the Persian Gulf War of 1991, for example, DNA analyses helped identify victims of the shooting and bombing. (A special DNA identification laboratory was established for this purpose.) And in 1996, after the explosion of Airline Flight #800 over Long Island, DNA testing was used to identify the remains of disaster victims. In that instance, VNTRs were used in the testing.
Genetic identifications have also brought to an end the concept of the Unknown Soldier. In 1998 scientists exhumed the remains of a serviceman entombed in Arlington National Cemetery and brought a bone sample to federal laboratories for analysis. Here they learned the identity of the soldier by comparing his DNA to that of a woman believed to be his mother. The soldier was an Air Force pilot shot down over Vietnam in 1972.
The idea of perpetuating the memory of an unknown soldier as a symbol of national remembrance arose after World War I when President Warren G. Harding dedicated the Tomb of the Unknowns on November 11, 1921. Sophisticated DNA analyses make it unlikely that the remains of any service personnel will go unidentified in the future. However, it is probable that unknowns from earlier wars will remain unidentified because relatives needed for matching blood samples have since passed away.
An identification of a different sort was made in 1992. Bones found in 1985 were believed to be the remains of Joseph Mengele, "doctor" at the Auschwitz concentration camp during World War II. (Mengele had disappeared after the war.) Family members maintained that the bones were Mengele's, and his mother and son agreed to give blood samples for DNA matching tests ("reverse" paternity test). Alec Jeffreys' laboratory conducted the DNA analysis, and he concluded that the DNA from family members matched that taken from the bones. The case was closed.
 In 1994 scientists identified the long-lost remains of Czar Nicholas II of Russia. The czar, Czarina Alexandra, four daughters (Olga, Maria, Tatyane, and Anastasia), a son (Alexis), and four others were executed by their Bolshevik captors on July 17, 1918. In 1991, a mass grave was found near Yekaterinburg, a Russian city where the murders presumably occurred. Then, in 1994, British DNA technologists extracted DNA from the bone cells, amplified it by the PCR, and compared its base composition with DNA obtained from blood cells of living family descendants (including Prince Philip of Great Britain, a grand-nephew of Czarina Alexandra). The base sequences of DNA were close enough to conclude that the remains were those of Czar Nicholas II and others. On July 17, 1998, eighty years after his death, the czar was given a state funeral in Russia.But the mystery was not completely ended. Czar Nicholas had a daughter named Anastasia, and soon after the executions, a number of women came forward proclaiming themselves Anastasia and demanding the czar's fortune. The most famous "Anastasia" was a woman named Anna Anderson, who maintained she was the czar's daughter until her death in 1984. In 1995, DNA technologists obtained a sample of Anna Anderson's intestinal cells (preserved by a hospital after surgery) and compared its nuclear DNA with the czar's. There was no match. When they compared the mitochondrial DNAs, the samples once again failed to match. The evidence indicates that Anna Anderson was an impostor. Indeed, the prevailing wisdom is that Anastasia perished with her family (despite the story in the 1998 animated movie). Parentage testing and Paternity Testing
Paternity Testing Theory
Who's the father? Paternity testing is used to determine whether a certain individual is the father of a particular child.
How does a paternity test work?Paternity testing is done in a DNA laboratory where DNA samples of the alleged father and the child are tested for possible match. DNA samples can also be taken at home and sent over mail to the laboratory - this is the so called home paternity test.
 Evaluation of paternity testing data usually proceeds by calculating the Paternity Index (PI) and the probability of paternity. The PI is computed via a likelihood ratio which weighs two probabilities – the probability that the alleged father s is the true biological father of the child c under the prosecution hypothesis Hp and the probability of another man i and not the alleged father s being the true biological father of the child c under Hd. Typically, DNA profiles from the alleged father, biological mother and the child are required for calculating the PI, although it can also be estimated without the mother’s profile. Let E denote all DNA evidence in a particular parentage case (i.e., DNA profiles from the child, mother and the alleged father). Then, by applying Bayes’ theorem to DNA evidence as described at the beginning of this chapter, we will have: This formula is virtually identical to that used for evaluation the prosecution and defense hypotheses under the logical model of evaluation of forensic evidence. The likelihood ratio:  in terms of parentage testing is called Paternity Index; it shows how many times more plausible the prosecution hypothesis is, given the DNA evidence. Usually, the numerator of this fraction, and consequently the probability of paternity under the prosecution hypothesis, will be equal to 1, while the denominator depends on the genotypes of the participants, which alleles are common between the alleged father, the child and the mother (if the mother’s sample is analyzed) and their frequencies in the population.
When the alleged father is not excluded from being the true biological father, the weight of the evidence toward this is the relative chance that he has transmitted the obligate allele (i.e., the allele which has to come from the alleged father if he is the biological father) to the child, when compared to an unrelated individual from the same population. Thus, PI for a particular locus expresses the relative chance of the alleged father transmitting the obligate allele versus an unrelated individual in the population. Buckleton and colleagues (2005a) and Balding (2005) present the formula used for calculating PI in duo and trio paternity cases taking into account various population genetic parameters as well as mutation events. Typically, PI is calculated for each locus tested and the CPI for the total number of loci N is calculated as a product of the individual PI:
 The probability of paternity Pr(P) is another representation of the CPI and is the probability of the alleged father being the true biological parent of the child. This index is calculated as follows:
 For a reasonable number of markers tested, the value of CPI usually exceeds 1,000. In non-criminal cases, a CPI of at least 100 (which corresponds to the probability of paternity of 99 per cent) is required for proving paternity in UK courts. The prior probability expression:
 evaluates the probability of paternity based on non-genetic evidence. Typically, in paternity disputes, the probability of both prosecution and defense hypotheses are considered to be equal to one-half. This practice is considered to be unfortunate by many eminent authors (for commentary see Balding, 2005). The alleged father may have had a sexual intercourse with the mother (possibly on more than one occasion) which increases his chances of paternity. If the alleged father had unprotected sexual intercourse with the mother around the time when the baby was conceived, the chances of him being the father will be even higher. Surely, if such evidence is available, the practice of assigning equal probabilities for Hp and Hd is misguided and even erroneous. However, no matter what the facts are, it is very difficult and even impossible for the jury to put a numerical value to non-scientific evidence. This is one of the pragmatic reasons for assigning equal prior probabilities to the alternative hypotheses.
Practical Paternity Test Since only two types of DNA must be compared, DNA profiles from the alleged father and the child, there is no need for child's mother to participate in paternity testing.In order to perform comparative DNA testing for paternity test, biological samples must be obtained from the father and child. Many laboratories often use buccal cell collection rather than drawing blood. Buccal cell collection involves wiping a cotton swab similar to a Q-tip against the inside cheek of an individual’s mouth to collect some skin cells. The swab is then dried or can be pressed against a treated collection card to transfer epithelial cells for storage purposes.
A simple buccal DNA collector may also be used for direct collection of buccal cell samples. A disposable toothbrush can be used for collecting buccal cells in a non-threatening manner. This method can be very helpful when samples need to be collected from children. After the buccal cells have been collected by gently rubbing a wet toothbrush across the inner cheek, the brush can be tapped onto the surface of treated collection paper for sample storage and preservation.
 The cheek, saliva or hair samples are sent to the DNA laboratory where they are treated in order to release the DNA coding from the cells. The DNA is then introduced to an enzyme that duplicates the DNA, producing millions of copies ready for analysis.The DNA profile is made up of STR (Short Tandem Repeat) markers. As a child inherits half of his or her DNA from each parent, every STR marker in the child's profile should be present in either the mother's or father's DNA. The laboratory will test at least 16 STR markers to determine paternity. The process is repeated several times with different probes, each identifying a different DNA area and producing a distinct pattern. Using several probes creates a greater than 99.9% certainty about paternity.With the availability of home paternity test kits you can easily collect the DNA samples at your home. You simply swab the inside of your cheek with one of the provided cotton swabs and then the child's cheek with the other. You then send the swabs back to the laboratory and the paternity results will be available typically in one week. Paternity test results are easy to understand and will provide conclusive answers. If the tested man is the true biological father of the child, the paternity test will show greater than 99.9% that he is the father (inclusion). If the tested man is not the biological father of the child, the report will show with 100% that he is not the father (exclusion of paternity). |