Ask a ScienceBlogger: Can forensics distinguish identical twins?

I have the honor of answering the next installment of "Ask a ScienceBlogger". This time around the question is:

Can a forensic scientist tell the difference between DNA from blood samples of identical twins?

The short answer is "No."

Identical, or monozygotic, twins occur when a fertilized egg divides into two embryos that then develop into separate fetuses. Because they come from a single egg that's been fertilized by a single sperm, identical twins have identical DNA. This makes it difficult for forensic scientists to distinguish between DNA from blood samples of identical twins.

To set up a DNA profile, forensic scientists analyze specific areas, or loci, of DNA. These loci, sometimes known as junk DNA, do not contain genes but contain repeated sequences of DNA called tandem repeats. Tandem repeats are a type of genetic variation, also called polymorphisms.

Polymorphisms in human DNA can be used to identify individuals just like conventional fingerprinting does. The scientists analyze multiple loci and generate a DNA pattern that is unique to each individual. This way, forensic scientists can compare, for example, a suspect's DNA profile with that found at a crime scene to find a match.

The large number of polymorphisms observed in the human genome means that it is virtually certain that every individual will have his/her own unique genetic pattern--with the exception of identical twins.

I spoke with Charity Davis, Ph.D., a scientist at the Georgia Bureau of Investigation, Division of Forensic Sciences, about DNA profiling, how it's done and how they deal with identical twins.

Dr. Davis says that currently forensic DNA analysis examines 15 loci plus one gender-determining locus. These 15 loci were originally picked by the FBI as regions that fit all the requirements for forensic DNA analysis. A gender-determining locus is further needed to confirm whether a subject is male or female, as there are cases where the gender is unknown.

"We examine the short tandem repeats in regions that are under Hardy-Weinberg and linkage equilibrium," she goes on to explain.

To be "under Hardy-Weinberg and linkage equilibrium" means the scientists assume the population subgroup from which the subject came is homogenous and mating randomly and that there is no selective pressure skewing the analysis.

Selective pressure would result in a certain DNA locus being more prevalent in a population and it would make it more difficult to observe differences in that locus.

But what about identical twins, who share those all-important polymorphisms?

"I believe there have been cases where one of [our] analysts has had to test identical twins because both were considered subjects" Davis says. " We were not able to detect any differences with the fifteen loci we examined. Because we can't tell a difference, we report that the DNA obtained from the evidence matches the person or his or her identical sibling."

There are other examples of DNA typing not being able to distinguish identical twins. This ABC News article tells the story of identical twin brothers involved in a prolonged legal battle over the paternity of a baby because DNA-typing methods were unable to distinguish between their DNA patterns.

Thanks to Dr. Charity Davis of the Georgia Bureau of Investigation for helpful discussions.
Reference: Medical Genetics. Second Edition. Jorde, Carey, Bamshad, White. 2000.
Image of monozygotic twins from http://www.mombaby.org/UserFiles/File/TTTS.html