Forensic and Paternity DNA Testing
Forensic and paternity DNA testing compares DNA profiles to establish identity or biological relationship. The method is short tandem repeat (STR) typing: PCR amplification of 15 to 24 highly variable loci (each with repeat units of 2 to 6 bases, differing in number between people), together with a sex marker (amelogenin), followed by capillary electrophoresis that sizes the amplified fragments.
Each person has two alleles at each locus, one from each parent, so a child must share one allele with the biological father at every locus except for mutations. Combining the loci gives a random match probability of 1 in 10 to the power 15 to 20 or rarer for unrelated people.
Additional tools are Y-chromosome STRs (paternal line, shared by male relatives), X-chromosome STRs (useful in sibship and grandparent testing), mitochondrial DNA sequencing (maternal line, for degraded samples and distant relatives), single nucleotide polymorphism panels and sequencing (kinship and ancestry, degraded samples), and non-invasive prenatal paternity from maternal plasma cell free DNA.
Uses in medicine include paternity and maternity testing, siblingship and other kinship testing (immigration, inheritance, repatriation), identification of remains in mass disasters, forensic evidence in sexual assault and criminal cases, chimerism monitoring after allogeneic haematopoietic stem cell transplantation (comparing recipient and donor STR profiles in blood and sorted cell fractions),
exclusion of maternal cell contamination in prenatal diagnostic samples, confirmation of sample identity in the laboratory (sample swaps), twin zygosity, and identification of donors in assisted reproduction.
Requests for paternity testing are common worldwide, with most tests showing the alleged father to be the biological father in about 70% of cases; the proportion varies with referral reasons.
Indications. Disputed or uncertain parentage for legal purposes (child support, custody, inheritance, immigration, adoption) or for personal knowledge. Identification of unknown remains. Sexual assault kits and trace samples (as ordered by courts or police). Confirmation of identity before transplantation or diagnostic procedures. Post-transplant chimerism assessment.
Maternal cell contamination exclusion before prenatal microarray or sequencing. Genetic counselling when non-paternity is suspected in the course of a genetic family study. Not to be carried out on a child without the consent of a person with parental responsibility, and legally valid results need a documented chain of custody.
Specimen. Buccal swabs from the child, mother (if available) and alleged father, collected by an independent professional who checks identity (photograph, identity document, signatures, fingerprints) and seals samples; alternatively EDTA blood (2 to 5 mL), blood spots on filter paper, or saliva. Home collection kits are suitable for informal testing but not for legal use.
Forensic material includes semen, blood, hair with roots, touch DNA from items, bone and teeth for remains; samples are dried, stored in paper (not plastic), and kept at room temperature or frozen. Prenatal paternity needs maternal plasma (10 mL), the alleged father's sample and a foetal fraction of at least 4 to 6%, or chorionic villi or amniotic fluid.
A form records the relationships, consent, ethnic groups and the reason for testing.
