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Science New blood test to determine paternity BLOOD TESTS have long been used as evidence in paternity cases if they exclude the accused male as the biological father. But H.B. 2941, now awaiting the governor's action, would allow as inclusionary evidence the use of the Human Leukocyte Antigen (HLA) test, a relatively new technique that is increasingly applied to paternity investigations. Since there are so few blood types, many people share the same type. Thus the courts usually admit blood test results only as exclusionary evidence, that is, when the tests indicate the accused father could not be the actual father because his blood type is incompatible with the child's. But the HLA test identifies over 3,500 different genetic combinations, thus fewer people share the same combination. This specificity suggests the test results can be used as inclusionary as well as exclusionary evidence. The test, available in most states, has received medical praise as the single most powerful tool in determining paternity probabilities. The HLA test concerns itself with chromosomes, of which each person has 46, grouped as 23 pairs. One chromosome of each pair is inherited from a person's mother, the other from a person's father. The HLA technique identifies specific locations (loci) on chromosome number six. These loci, called HLA-A and HLA-B, control the histocompatibility antigens — substances which determine if a tissue transplant will be accepted or rejected. Each locus produces two antigens, so four antigens constitute a "full house." Other loci have been identified, but most attention has been given to HLA-A and HLA-B. Each chromosome has an "A" antigen and a "B" antigen which are inherited as a unit, called a haplotype. For example, one chromosome in pair number six may contain the antigen A-1 coupled with B-39, while the other chromosome may have antigen A-23 teamed up with antigen B-21. A child inherits a particular A/B combination from his mother, and he gets the other chromosome with a particular A/B combination from his father. Over 3,500 different haplotypes have been identified. In a paternity dispute, the HLA method uses lab techniques to identify the antigens found in the mother, child and presumed father. By then applying laws of heredity, it can be determined if it is even genetically possible for the accused father to actually be the father. If it is found that the child and the accused father have a haplotype in common, statistical methods are used to assign probabilities of paternity. Probabilities are determined from tables which list haplotype frequencies for a given ethnic group and geographical region. For instance, one of the more common haplotypes — A2, B12 — occurs in about four out of every 1,000 whites. Most haplotype frequencies were determined in 1972, when scientists went to all regions of the world and collected samples for antigen identification. Some interesting findings turned up. Data showed that antigen 36 is nine times more common in blacks than whites, while antigen 42 is found only in blacks. The data were of interest to anthropologists as well, for they suggested possible lines of descent. Strong similiarities in haplotype frequencies found between Eskimos and New Guinea natives support the popular Bering Strait migration theory. The HLA test is one of seven blood tests recommended for. paternity disputes through guidelines established by the American Bar Association and American Medical Association. When performed by itself, the HLA technique offers 90 percent probability of exclusion; in conjunction with the other tests, exclusion probabilities exceed 95 percent. Determining the probability of inclusion is trickier. An estimate of the probability of the presumed father actually being the father is made by subtracting from 100 percent the probability of another male contributing the same haplotype. For example, tests show that John Doe, an accused father, has a haplotype in common with the child. This haplotype, A-2, B-12, occurs with a frequency of about 0.4 percent, meaning that 4 out of every 1,000 males will have this same haplotype. Since every person has two haplotypes, there is a 50-50 chance of any male in this group contributing the haplotype found in the child, so the 0.4 percent must be divided by two. This 0.2 percent is then subtracted from 100 percent, yielding 99.8 percent — the probability that John Doe actually is the father.
The technique is not fail-safe. Despite a tight chain of custody of the blood samples, internal quality controls and strict government standards, mistakes can happen. Blood samples can be mislabeled, results can be misinterpreted, and good sera (used to identify the antigens) can be hard to come by. McConnachie said, "When you're dealing with a situation where you're asking a man to take responsibility for a child, you need to consider all aspects." Because of these reservations, McConnachie was relieved by the last clause of the bill which stipulates that, "... the results of blood tests alone shall not be sufficient grounds for determining that the man is the father of the child." This clause, according to McConnachie, provides an important "out" because "there is always that chance that some other man, somewhere, has that [same] haplotype and could have contributed it to the child ... it is up to the courts to assure that any probabilities presented will stand up to scrutiny." The bill, sponsored by Rep. Helen Satterthwaite (D., Champaign), was signed by Gov. James R. Thompson on September 4 and becomes effective January 1. Support for this column, which reports policy developments concerning science and technology, is provided in part by a National Science Foundation grant to the Illinois Legislative Council Science Unit, where Julie A. Dutton is a research associate. October 1980/Illinois Issues/27 |
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