What is a Single Nucleotide Polymorphism (SNP) in Forensics?
A Single Nucleotide Polymorphism, or SNP (pronounced “snip”), is a variation at a single, specific position in a DNA sequence. It’s the most common type of genetic variation among people. For example, at a particular spot in the genome, most individuals might have a Guanine (G), but a minority of the population has an Adenine (A) instead. This G/A variation is a SNP. In forensic science, SNPs are powerful tools for predicting ancestry and physical traits from a DNA sample, especially when traditional analysis is challenging.
What Makes a SNP?
SNPs are essentially point mutations that have become common within a population (typically occurring in at least 1% of people). While most genetic variation is based on longer repeating sections (like STRs), SNPs are changes to a single “letter” of the DNA code. The human genome contains millions of these variations, creating a rich source of information about an individual’s genetic makeup.
How SNPs are Used in Investigations
While not typically used for primary identification like STR analysis, SNPs provide valuable investigative intelligence, particularly in cases with unknown suspects.
- Biogeographical Ancestry Estimation: Certain SNPs are found in different frequencies among populations from different parts of the world. By analyzing a panel of these Ancestry Informative SNPs (AISNPs), a lab can provide a statistical estimate of an individual’s ancestry (e.g., European, African, East Asian, etc.), providing a valuable lead for investigators.
- Forensic DNA Phenotyping (FDP): This cutting-edge technique uses SNPs to predict Externally Visible Characteristics (EVCs). By analyzing SNPs located in or near genes associated with pigmentation, scientists can predict a person’s likely eye color, hair color, and skin tone from their DNA alone.
- Analysis of Degraded DNA: A SNP is the smallest possible unit of variation—just one base pair. This makes it an extremely small and robust target for analysis. In cases involving old or environmentally damaged evidence (like skeletal remains), where the DNA is heavily fragmented, longer STR markers may be destroyed. However, the short segments required for SNP analysis are much more likely to survive, making it an excellent tool for degraded samples.
