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Interpreting Paternity Tests

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The Bascis

The GTL Report shows the results of laboratory DNA tests that provide evidence regarding the alleged family relationship. This is a brief explanation of the meaning of the numbers and other items that appear in a DNA test report.

The laboratory tests the DNA isolated from buccal (cheek) swabs to locate certain regions of chromosomes that vary in length between individuals. Sixteen of these sites are tested, each site is called a "locus", ("loci" - plural). Analysis of these 16 loci in a large population has revealed many different sized fragments, or alleles, associated with each locus. These are the genetic markers* used in parentage analysis.

Because each individual has two of each type of chromosome, one inherited from each parent, everyone has two alleles at each locus. These two alleles are sometimes identical (homozygous), but usually they are not the same size (heterozygous). During parentage testing, the laboratory identifies the length of the two alleles found at each locus

The Details

The report you will be given shows numbers (in the first column) that indicate each of the 16 loci involved in the testing process.

The columns marked "allele" on the report contain numbers indicating the two alleles found at each locus (or one number if they are the same size).

If, for example, a child has two alleles that are designated 12.1 and 18, and if the mother has alleles 12.1 and 16, then the child inherited the 12.1 allele from the mother. The child has to have inherited the 18 allele from the father. The 18 allele is the "obligate paternal allele." Generally, the alleged father must have this allele if he is the biological father of the child.

Fifteen different loci are used as genetic markers in the tests, as well as one (Amelogenin) to confirm the sex of the person providing the sample. If the alleged father does not have the matching allele at every tested locus, then he usually cannot be the biological parent. If he does have it, then he could be the father. A "relationship index" (called the "Direct Index" in the report) for each locus is calculated based on information including the portion of the male population that has the obligate paternal allele at that locus. This index is reported for each DNA locus.

A combined relationship (or "Direct") index for all of the tested alleles is then calculated and appears below the chart. This number is used to calculate the "probability of relationship," which is the percentage likelihood that a man with the alleles of the alleged father is the biological parent of the child, as compared to an untested, unrelated man of the same race. If the DNA of the alleged father is consistent (to a degree of mathematical certainty) with that of the child, then the report will conclude that the alleged father cannot be excluded as the biological father of the child. If the DNA is not consistent, it will conclude that the alleged father can be excluded as the biological father of the child.

* Each locus used in the testing is composed of a variable number of repeating short sequences of the bases A,C,T, and G; such as ACGACGACGACG. Each allele has a different number of repeats giving rise to a different fragment length amplified by the polymerase chain reaction (PCR).

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