REPORT OF RESULTS OF SIBSHIP TESTING OF FOUR ANONYMOUS MOTHER-CHILD PAIRS

GENELEX FILE NO. GL 8337
July 28, 1997
Prepared by GeneLex Corporation
12277 134th Ct. NE Redmond, Wa 98052
425-825-2850

EXECUTIVE SUMMARY SIBSHIP TESTING OF FOUR ANONYMOUS MOTHER-CHILD PAIRS

Draft July 16, 1997

We have completed RFLP DNA testing using generally accepted methods on four pairs of mothers and children. This testing was performed in order to determine how many, if any, of the children might have the same father or fathers. The samples were provided to us by the DNA Diagnostics Laboratory at the Oregon Health Sciences University, and identified to us by number and relationship only. The testing that was performed demonstrates that the children of pairs one, three and four have the same father, and that the child of pair two has a different father. The probability that this interpretation of these test results is correct exceeds 99.99%.

The analysis consisted of using DNA probes to examine the DNA profiles of the test subjects at twelve different genetic locations (loci). These particular loci were chosen because they vary a great deal from person to person and have been widely used in forensic and paternity DNA testing. Following the production of the primary data consisting of autoRADS (autoradiographs), generally accepted genetic, statistical and population genetic techniques were used to generate a series of analyses which are reported here.

In general, the more genes that two children have in common, the more likely it becomes that they have a common parent. The basic laws of genetics determine which of the two versions (alleles) of the genes, found at each locus in the child, is the allele that must have come from the father. This is the obligate paternal allele. It is identified by comparing the childŽs DNA profile to the motherŽs. What the mother didnŽt contribute had to come from the father.

Since the father has only two versions of each gene, one or the other of them must appear in each of his children. Any two children that have the same father are expected to also have the same paternal gene at approximately one-half of the loci tested. If three children have the same father, at least two of them must share the same paternal gene at each locus. Whenever a third version of the gene appears it means that there is more than one father, or that a mutation has occurred.

When the paternal genes have been identified the techniques of population genetics are used to estimate how common or rare that particular gene is in the population. This is accomplished by looking in population databases made up of the DNA profiles of hundreds of individuals. In practice these population databases are collected, based on the race or ethnic affiliation of the individuals making up the database. In this particular case the Caucasian database was used because the racial or ethnic affiliation of the fathers of the children are not known. The use of other racially or ethnically derived databases would alter the numerical values of the reported results, but would not alter the conclusions that can be reached from this testing.

The gene frequency data obtained from the population databases is used to calculate the likelihoods and probabilities of half-sibship or unrelatedness of a series of comparisons between different combinations of the tested children. The results of each of these thirteen comparisons is contained in the accompanying reports. The reported likelihoods are a numerical value that estimates the chance that the tested children could have the same father. A likelihood less than one suggests that the individuals do not have the same father. A likelihood greater than one suggests that they do. The likelihood ratios calculated for each locus are combined by multiplying them together to produce the combined likelihood. The more genetic testing that is done, the greater our confidence in the results, and the higher the likelihood of half sibship ratio, the greater the probability that the tested children have the same father.

According to the verbal predicates established by a joint committee of the American Bar Association and the American Medical Association for standard parentage testing a likelihood of 100 means that paternity is ¿extremely likely�E A likelihood greater than 500 puts the test results into the ¿practically proven�Ecategory. (Journal of Family Law ...1976)

Circumstances under which these test results could be in error include the possibility that instead of C1, C3 and C4 having the same father they were fathered by identical twin brothers.

Table 1.
Shared obligate genes at twelve polymorphic loci. Matching alleles within each row are given the same letter designation. The numbers under the letters represent estimates of the sizes of the alleles in base pairs. Matching alleles are ones that differ in size by less than two per-cent. The presence of two possible paternal genes in Child 1 at the D7S467 locus indicates that both bands in the mother and child matched by chance; making it impossible to distinguish which is the true paternal gene. Shaded areas represent matching alleles at each locus.

Summary of Reported Test Results

The likelihood ratio for half-sibship indicates how many more times it is estimated that these test results would occur with children who are half-siblings than by chance. The probability of half-sibship is derived from the likelihood ratio and assumes a 50% prior chance. The assumption of a prior chance is a requirement of the equation that is used to produce the probability of half-sibship. If a different prior chance were used the probability would be slightly different.

The likelihood ratio for unrelatedness indicates how many more times it is estimated that these test results would occur by chance with children who are unrelated than half-siblings. The probability of unrelatedness is derived from the likelihood ratio and assumes a 50% prior chance. If a different prior chance were used the probability would be slightly different.

The zero value for the likelihood ratio and probability of half-sibship excludes the possibility that Child 2 is a half-sibling of the others.

Each possible pair of the children is assumed to be half-siblings and the third tested to see if their father is the same as the other two. The comparison of Child 3 and 4 with Child 1, because it produces the lowest likelihood of half-sibship, leads to the second most plausible explanation for the data, that Child 1 is unrelated. The data is at least 40,000,000 times more likely the result of all three having the same father than if any one of them is unrelated.