Forensic science laboratories may be involved in paternity and relatedness cases. In these cases, DNA evidence is generally interpreted using likelihood ratios, comparing probabilities of the evidence under alternative propositions.^01, 03

DNA parentage and relatedness testing can be conducted in a forensic science laboratory to

• Resolve questioned paternity
• Assist investigation of alleged rape or incest
• Assist in the identification of a missing person or unidentified remains
• Determine other familial relationships, such as maternity or sibling relatedness
• Discriminate between identical and fraternal twins.¹⁹

The rules of parentage testing are as follows:

1. The child cannot have a genetic marker that is absent in both parents.
2. The child must inherit a pair of genetic markers from each parent.
3. The child cannot have a pair of identical genetic markers, unless both parents have the marker.
4. The child must have the genetic marker, if that marker is present as an identical pair (homozygous) independently in both parents.¹⁹

The above are conditional on no mutation having occurred. Analysts need to be aware that inconsistencies at a locus could be due to a mutation. Mutations are rare events, and in general, two inconsistencies are sufficient to exclude in paternity cases. This module will not address statistics involving mutations. Note that the American Association of Blood Banks provides formulas that account for the inclusion of mutations into statistical paternity calculations.

To determine if the alleged father is the true biological father, the DNA profiles of the child, mother, and alleged father are compared. A child inherits two different alleles at each genetic locus—one from the mother and one from the father. If a child has an allele that the mother does not have, this obligate allele has to come from the biological father.²⁰ The results are either an exclusion —the alleged father is not the biological father—or an inclusion.¹⁷

If the alleged father has the same allele as the obligate allele, a Paternity or System Index (PI or SI) can be calculated. This is the relative probability that the alleged father and not an unrelated, randomly selected male of the same ethnic background transmitted the obligate allele to the child.

This is a likelihood ratio and is presented in the formula X | Y, where X is the chance that the alleged father could transmit the obligate allele and Y is the chance that an unrelated man of the same race could have the allele.²¹ X is assigned the value of 1 if the alleged father is homozygous for the allele of interest and 0.5 if the alleged father is heterozygous.

The probability of an unrelated, randomly selected man possessing the obligate allele is determined by using a database that lists the frequency distribution of individual alleles. If there is more than one obligate allele, the individual paternity indexes can be multiplied and the total across all loci is called the Combined Paternity Index (CPI). This is a measure of the strength of the genetic evidence and is an odds ratio, not a probability.²⁰

An interpretation of the CPI is as follows:

• CPI can range from 0 to infinity.
• If CPI is between 0-1, the genetic evidence is more consistent with non-paternity than paternity.
• If CPI>1, the genetic evidence is more consistent with paternity than non-paternity.¹⁶

It is normal practice to establish a threshold value for CPI, above which it is accepted that the tested man is the true biological father. This threshold is 1000 in Europe, but can be as low as 100 in the USA.¹⁹

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