How is paternity determined in a DNA test?

The genetic loci analyzed contain DNA sequences known as short tandem repeats (STRs)—short, repeating units of DNA.  For each person, the number of repeats present at each genetic locus is variable and is inherited from their biological parents.  For each genetic locus, each person has two genetic markers, or alleles (alternate forms of the DNA sequence):  one allele is inherited from their biological mother and one allele is inherited from their biological father.

The test result report lists the allele sizes (number of repeats) for the mother (if tested), child, and alleged father at each genetic locus.  By testing the mother, we can determine which allele the child inherited maternally.  The allele that the child received from its biological father (called the obligate paternal allele) can therefore be determined by subtracting the mother’s contribution to the child’s DNA.  If the alleged father does not possess the obligate paternal allele at three or more genetic loci, then he is excluded (ruled out) as the biological father.  If only the child and alleged father are tested, and they do not share any alleles in common at three or more loci, then the alleged father is also excluded.  When the alleged father possesses the obligate or possible paternal alleles at all loci tested, then he is not excluded (not ruled out) as the biological father.  Based on these genetic findings, a probability of paternity is calculated and reported.

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