Genetic Identity Calculators

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Genetic identity refers to the unique DNA profile that distinguishes one individual from all others (except identical twins). It is established by analyzing highly variable genetic markers — typically short tandem repeats (STRs) — at multiple genomic loci. The probability that two unrelated individuals share the same profile at all analyzed loci is astronomically small (typically < 1 in 10¹⁵). Genetic identity analysis is used in forensic casework, paternity and kinship testing, disaster victim identification (DVI), immigration cases, missing persons investigations, and population genetics research.

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STR (Short Tandem Repeat) Profiling

STRs are repetitive DNA sequences (e.g., AGAT repeated 8–20 times) with highly variable repeat numbers between individuals. The US CODIS (Combined DNA Index System) requires 20 core STR loci for criminal databases. For each locus, each person has 2 alleles (one per chromosome) — the combination across all loci forms the DNA profile. PCR amplifies STR loci using fluorescently labeled primers; capillary electrophoresis separates alleles by size.

Match Probability

The probability of a random unrelated individual matching a profile = product of genotype frequencies across all loci (assuming Hardy-Weinberg equilibrium and independence between loci). For 20 CODIS loci, match probability is typically < 1 in 10¹⁵ — far below the world population (8 × 10⁹). This is expressed as the random match probability (RMP) in forensic reports.

Paternity Testing

Paternity index (PI) = P(alleged father is biological father) / P(random unrelated man). A PI > 100 at each locus and combined PI > 10,000 is generally accepted as strong evidence of paternity. The probability of paternity (PP) = PI / (PI + 1) — typically > 99.9% with 15–20 STR loci for true paternity.

Mixture Interpretation

DNA mixtures (e.g., from crime scenes with multiple contributors) require probabilistic genotyping software (TrueAllele, STRmix) to estimate the probability of observing the mixture given different contributor combinations.

Glossary

STR (Short Tandem Repeat)
Highly variable repetitive DNA sequences used for identity testing; allele = repeat number at a locus; 20 CODIS loci give match probability < 1 in 10¹⁵ for unrelated individuals.
Random Match Probability (RMP)
The probability that a randomly selected unrelated individual has the same DNA profile; calculated as the product of genotype frequencies across all loci; typically < 10⁻¹⁵ for 20 STR loci.
Paternity Index (PI)
Likelihood ratio comparing probability of observed child genotype if alleged father is vs. is not the biological father; combined PI > 10,000 across all loci strongly supports paternity.

Frequently Asked Questions

A DNA profile is a set of numerical allele designations at multiple STR loci. STRs are tandem repeat sequences where the repeat unit is 2–7 bp; alleles differ in repeat number. To create a profile: extract DNA from sample; PCR amplify 15–20+ STR loci simultaneously using fluorescent multiplex kits (GlobalFiler, PowerPlex Fusion); separate alleles by size using capillary electrophoresis; compare peaks to allelic ladder. The result is a series of numbers (e.g., 14,16 at one locus) forming a unique numerical profile. Match probability across all loci is typically < 1 in a quadrillion.

Paternity testing compares the child's DNA profile to the alleged father's at 15–20+ STR loci. The child inherits one allele at each locus from each parent. If the alleged father is the biological father, he must have one allele matching each of the child's paternal alleles at every locus. The paternity index (PI) quantifies the likelihood ratio of paternity vs. random man at each locus; the combined PI (product across all loci) typically exceeds 1,000,000 for true paternity. Probability of paternity = PI/(PI+1) > 99.9999% establishes paternity.

CODIS (Combined DNA Index System) is the FBI's national DNA database containing STR profiles from convicted offenders, arrestees, and forensic evidence. The US CODIS requires 20 core STR loci since 2017 (expanded from 13). Crime scene DNA profiles are searched against CODIS for cold case investigation — a match to a database profile ('cold hit') links a crime to an individual or to another crime scene. CODIS contains >20 million profiles. The probability of a false match is designed to be < 1 in the world population using 20 STR loci with stringent match probability calculations.

Disaster victim identification (DVI) uses DNA profiling to identify remains from mass casualty events (plane crashes, natural disasters, terrorist attacks). Direct comparison: DNA from remains is matched directly to ante-mortem (before death) samples (toothbrush, stored blood, tissue). Kinship identification: when ante-mortem samples are unavailable, DNA from family reference samples (parents, children, siblings) is used to identify remains via kinship analysis using likelihood ratios. A combined likelihood ratio >10,000 typically provides sufficient evidence for identification. International standards are set by INTERPOL's DVI protocols.