DNA profiling is considered the gold standard for individual identification. DNA-containing bodily fluids found at crime scenes can often be linked to the perpetrator with a high degree of accuracy, often measured in one per billions. It is highly individual and therefore highly accurate for identifying a given individual.
But since identical twins begin as the same fertilized egg, they have identical genetic material (DNA). After fertilization, the fertilized egg divides into two cells. To produce identical twins, these two cells separate and then each progresses forward to produce an individual. This results in two identical individuals with identical DNA. Or does it?
Standard DNA testing uses the concept of Short Tandem Repeats (STR’s). STR’s are simply short segments of DNA that repeat in certain areas of the very long DNA strand found in all of us. The number of these repeats in the various locations are what allow DNA profiling to distinguish individuals so accurately. This is a complex, though not really difficult to understand, technique which is discussed in great detail in two of my books: Forensics For Dummies and Howdunnit: Forensics.
But scientists have known for years that the DNA of identical twins is not perfectly identical. It might or might not start out that way at that first cell division but for sure as the cells divide and the individual grows within the uterus, minor DNA changes can occur. These are on the level of the base pair sequences that make up the DNA chain.
Another DNA technique called Single Nucleotide Polymorphism (SNP) actually looks at each base in the DNA strand and uses this for comparison with another strand to determine if they came from the same individual. This is the direction that DNA testing is going but for now STR remains the method of choice.
Identical twins would look the same using STR analysis but a deeper analysis using SNP would reveal variations, thus allowing identification and separation of two identical twins. Let’s say, blood is left at a crime scene and that blood is matched to a particular individual. Let’s further say that this individual is an identical twin. STR DNA analysis would not distinguish between these two brothers, But if SNP is employed, the one who left the blood at the scene can be distinguished from his identical twin.
The recent French serial rape investigation involving identical twins Yohan and Elwin would be a case in point. Applying the SNP technique in this situation would likely solve the case.
Pretty cool stuff.
From HOWDUNNIT: FORENSICS:
SINGLE NUCLEOTIDE POLYMORPHISM
Single nucleotide polymorphism (SNP) is a new technique that will likely see increased use in the future. The major problem at present is that it is expensive. We saw that RFLP fragments were fairly long, a drawback that lessens their value in degraded or damaged samples (discussed later). This problem was circumvented by the discovery of STRs, which are very short fragments. But, what if the DNA examiner could use single nucleotide bases as the standard for matching? This would increase the discriminatory power of DNA even further. This is what SNP does.
Let’s say that two sequenced DNA strands looked like this:
CGATTACAGGATTA and CGATTACAAGATTA
If we searched for an “ATTA” STR repeat, these two strands would be indistinguishable
since both have two ATTA repeats. But, with single nucleotide analysis the strands differ by a single base: The ninth base in the first sequence is guanine (G), while it is adenine (A) in the second one. SNP can be used with restriction enzymes in the RFLP technique, or with PCR, where it can be easily automated. Theoretically, this will allow for discriminating two DNA samples based on a single nucleotide difference.