|MadSci Network: Genetics|
Hello Amie, Basically, it comes down to what you consider a match or an inclusion, and this question involves math and statistics. In the case of matching an individual to a source of blood at a crime scene, forensic scientists obtain a DNA profile from the bloodstain and the suspect's blood standard and compare them. If they are different, he or she is excluded as a source of the stain. If they match, then the question is, how often does this profile occur in the general population. Studies have been conducted on DNA profiles obtained from hundreds of people from different population groups to obtain frequency charts. A good discussion of this material can be found at: http://www.fbi.gov/programs/lab/fsc/backissu/july2000/source.htm Your fictional case is a little different than the forensic case described above. Paternity testing ivolves analysis of the gentic relations of child, mother, and putative father. Mutation is a factor to be taken into account that is not an issue in forensic testing. According to the "Standards for Parentage Testing Laboratories", "an opinion of non- paternity shall ordinarily not be rendered on tha basis of a single indirect exclusion or on the basis of an exclusion at a single DNA locus". DNA loci used for comparison are chosen because they are highly polymorphic; however, they are highly polymorphic because they are mutable. In cases brought to establish paternity for child support, inheritance, custody, and other purposes, the law gives the claims of the parties roughly equal weight and uses a civil, rather than the criminal, standard of proof. In paternity cases, DNA profiles of the mother, child and putative father are obtained and compared. If the putative father is not excluded, a "paternity index" is calculated which is a likelihood ratio - the probability of the mother-child-father profile combination if the putative father is the true father divided by the probability of this combination if a randomly selected man is the father. In your fictional case, if one of the three suspected men truly was the father, two of the three men would immediately be cleared using DNA typing. The other man's DNA would include him as being the father. Typical results of paternity typing are on the order of 0.9901 to 0.9999. Depending on the DNA expert witness, they may testify that this individual is the father to the exclusion of all other individuals. I hope that this information helps and good luck with your project. Dale L. Laux Forensic Scientist
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