Understanding How Twin DNA Testing Works

Paternity and Forensic Twin DNA Testing

 Twins are often stereotyped as being clones and are expected to be “identical”. A common question posed in the paternity and forensic field is, “Do twins have the same DNA?” Prior to answering this question, it is important to know the difference between fraternal twins and identical twins.

Fraternal Twins & Their DNA

Fraternal Twins DNA DNALABDizygotic twins are what people more commonly know as fraternal twins. They are the result of two separate eggs being fertilized by two different sperm (1). As a result, fraternal twins are two separate individuals with each having their own unique DNA. Just like any other siblings that are born at separate times, dizygotic twins share approximately 50% of their genetic traits (1).

Unlike identical twins, it is rare, however possible, that fraternal twins have two different fathers. This process is known as superfecundation (2). Superfecundation occurs when sperm from different partners fertilize different eggs from the same menstrual cycle (2). There have been cases where legal paternity tests have revealed superfecundation and the results were presented in court for child support decisions (2). The ruling of one court decision in this type of case was for the alleged father to support only the child conceived by him, and the second alleged father to support the second child (2).

Standard DNA testing protocols available in the paternity and forensic fields allow for fast, easy and affordable differentiation between fraternal twins. Contact TheDNALAB customer service for any inquiries regarding a twin DNA test booking or information!

Identical Twins & Their DNA

Identical Twins DNA DNALAB

Monozygotic twins are more commonly known as identical twins. They are the result of a single fertilized egg which splits into two parts after conception and forms two individual embryos (1). Since the two embryos have split from a single sperm/egg combination, monozygotic twins are considered genetically identical and cannot be differentiated using standard DNA testing protocols (1-3).

With standard DNA testing protocols, the inability to differentiate identical twins as alleged fathers or as sources of DNA found at a crime scene may cause problems with paternity and forensic cases (3). There have been some cases in which DNA test results have been inconclusive in the paternity and forensic field.

How to Differentiate Identical Twins

It is most commonly believed that identical twins share identical DNA, however, with new advanced technology and developing methods this belief can be proved not necessarily true. The standard DNA testing protocols only look at a small portion of an individual’s DNA. With new advanced technology such as next generation sequencing, we have the potential to analyze the entire human genome, which can help differentiate between identical twins. This differentiation is done by identifying at least a single mutation where the DNA of one identical twin differs from the other (4).

Mutations are basically “errors” or “typos” in the human DNA which occur during development (4). These mutations are completely random, which makes it very unlikely that both twins would have the same mutations (4). Hence, looking at even a single mutation can help conclude a paternity or forensic case. Currently, the process of analyzing the entire genome is very expensive and time consuming, therefore, it is not commonly offered by DNA testing facilities.

How Fingerprints Can Help

Fingerprints DNA Testing DNALAB

Identical twins can also be differentiated by their fingerprints. Genetic and environmental factors contribute to the unique formation of everyone’s fingerprints, including identical twins (5). The shapes, ridges and patterns that define each unique print are developed within the womb based on movement and positioning of each foetus during development (5). When examining a crime scene, viable fingerprints are not always retrieved from a crime scene and therefore it may not help with the differentiation of identical twin suspects.

Other Characteristics In Forensic Cases

Evidence presented in court, aside from DNA evidence, can also help distinguish one identical twin from the other. For instance, a witness may have observed a unique characteristic feature, such as a tattoo, which only one twin has and not the other. Video surveillance and other circumstantial evidence presented in court can also help potentially identify one identical twin from the other.

To date, there have been a few paternity and forensic related cases around the world where identical twins could not be differentiated through DNA testing. In some forensic cases, no charges were laid since DNA testing was unable to determine which twin may have been involved in the crime, and the lack of other evidence to distinguish one from the other.

A Look at Twin DNA Cases In The News

DNA News Stories Twins

Fatal Home Invasion – The Saia Brothers

A 2012 fatal home invasion in Saint John, New Brunswick became complicated when evidence found from the crime scene linked the DNA to identical twin brothers. Brandon and Bradley Saia, were both accused of breaking into a house with faces covered using masks, as well as carrying weapons (6). Two victims were stabbed during this incident; one of whom passed away in hospital (6).

DNA found on a bandana and two sets of gloves that were left behind at the scene yielded a suitable DNA profile, which could be compared to a known individual. In this case, the forensic laboratory was not able to distinguish whether the profile obtained was from both brothers or just one brother, nor could they tell which one it may be (6). This case however, did not solely rest on DNA evidence. Aside from DNA, there were two key witnesses who also provided statements. The prosecution told the court that placing a single suspect at the scene was not needed for a conviction, since witnesses had provided statements that both brothers were involved in the planning of this crime (6).

After a few hours of deliberations, the jury rendered their verdict and both identical twin suspects were found guilty of all three charges against them. This goes to show that even though standard DNA testing protocols may not be able to distinguish one identical twin from the other, other evidence presented during a trial may still help a jury to reach a verdict.

Sexual Assault – The Cooper Brothers

A Michigan sexual assault case from 1999 also faced the issue of identical twin DNA testing. In this case, the victim was sexually assaulted by her attacker from behind (7). The assault was reported, and a sexual assault examination was conducted. The case came to a standstill with no fingerprint evidence and no suspect description from the victim (7).

The crime lab processed the evidence from the sexual assault examination and the DNA profile obtained led to a suspect. Jerome Cooper was the suspect whose DNA matched the DNA found on the assault victim (7).

The catch? Cooper had already been in prison since 2002 for an unrelated home invasion verdict (7). After further investigation, authorities discovered that Jerome Cooper had a twin brother, Tyrone Cooper (7). Both brothers had a previous sexual assault record, and DNA comparison revealed that they were identical twins (7). With both brothers denying the offence and with a lack of evidence, this case remains unsolved. Perhaps advanced technology and methods can one-day help solve this type of inconclusive case.

Twin Accused of Killing Twin – The Ali Brothers

Identical twin DNA can cause dilemmas when one twin is accused of killing the other. Wael Ali was accused for the murder of his twin brother, Wasel Ali, in Maryland (8). The investigators believed that both brothers had gotten into a fight, and Wael strangled his twin brother to death (8).

Prosecutors could not confidently place Wael at the scene of the crime as there were no fingerprints located and there was no DNA evidence to identify one twin from the other (8). The jury in this case failed to reach a verdict and this case resulted in a mistrial (8).

Sexual Assault – The McNair Twins

In 2004, two women were abducted and sexually assaulted nine days apart from one another (8). DNA profiles obtained from the victims matched the DNA of twin brothers, Dwayne and Dwight McNair (8). During this period, due to the lack of DNA evidence to differentiate between the two identical twins, no charges were laid.

Investigators, however, decided to apply new forensic technology to help identify genetic differences between the two twins (8). By looking at regions of DNA where mutations had occurred and comparing it with the DNA evidence, it was concluded that Dwayne was “2 billion times more likely to have been the source of the DNA evidence than his brother” (8). However, the judge ruled that this scientific technique is new and not supported by enough research to its accuracy and reliability (8).

Paternity Battle of the Miller Twins

Twin brothers from Missouri, Raymon and Richard Miller, faced the challenges of distinguishing their DNA apart during a paternity battle (9). Both identical twin brothers were unknowingly seeing the same woman, who testified that she had intercourse with each of the brothers on the same day at different hours (9). Raymon was claimed as being the father of the child by the mother, however he demanded a paternity test for his twin brother Richard as well.

As mentioned previously, a standard paternity DNA test was not able to help in this scenario. The test concluded that there was over 99.9 percent probability that each these twin brothers were the alleged father in question (9). Neither of the twins were willing to pay child support, however Judge Copeland ruled Raymon would remain the legal father of the child (9). The Judge’s decision was not solely based on DNA evidence. The testimony provided by the mother was also taken into consideration (9). Raymon continues to oppose the decision and hopes to keep appealing it.

Fingerprints Solved the Crime – The Smith Twins, Donald and Ronald.

Donald Smith was arrested for carjacking and shooting a pre-school teacher in 2008 (10). The arrest was made based on eyewitnesses, video surveillance and DNA results from forensic evidence (10). Donald claimed he was innocent of this crime, and it was his identical twin, Ronald Smith, who was guilty.

Once again, since the suspects are both identical twins, the DNA evidence linking Donald to the crime does not have much power. If this case relied only on DNA evidence, then either one of the twins could have committed the crime. In this particular case however, fingerprints were collected at the crime scene, which acquitted Donald, and incriminated Ronald (10).


Future of DNA Testing DNALAB

What Is the Future of Identical Twin DNA Testing?

As seen through some examples, identical twin DNA can lead to problematic issues, especially when DNA testing is an important factor for legal and/or criminal cases. When the evidence is not unique to an individual, the power of placing the individual at a specific location is greatly diminished. Developments in the forensic science field, crime scene analysis and DNA profiling have been very important for the justice system to help prosecute offenders and eliminate innocent individuals.

The challenge of differentiating the DNA of identical twins has encouraged the development of reliable technologies which can possibly be used by criminal investigators or the general public (for paternity cases) to have conclusive results. There have been some breakthroughs with the innovative technologies, however they continue to require more research around their reliability and accuracy. Even though criminal and legal cases involving identical twins are rare, they will continue to prove challenging and allow perpetrators to evade justice until science can further address the issues of identical DNA.

Book your Twin Test today!

Determining if twins are fraternal or identical is not always obvious. Identical twins do not necessarily always have the same physical characteristics; for instance, different eye and hair colour are possible. Twin curiosity tests are available, easy to process and affordable to determine the type of zygosity (fraternal or identical). If you are interested in getting a Twin DNA Test or have any other questions regarding DNA Testing in Canada, please contact TheDNALAB customer service department.

  1. Fierro PP. Overview of Identical Twins [Internet]. 2017 [cited 2017 Oct 23];Available from: https://www.verywell.com/identical-twins-2447126
  2. Weller C. NJ Twins Have Different Fathers Due to Rare, But Not Impossible ‘Superfecundation’ [Internet]. 2015 [cited 2017 Oct 17];Available from: http://www.medicaldaily.com/nj-twins-have-different-fathers-due-rare-not-impossible-superfecundation-332562
  3. Weber-Lehmann J, Schilling E, Gradl G, Richter DC, Wiehler J, Rolf B. Finding the needle in the haystack: Differentiating “identical” twins in paternity testing and forensics by ultra-deep next generation sequencing. Forensic Science International: Genetics [Internet] 2014; 9:42–6. Available from: http://www.fsigenetics.com/article/S1872-4973(13)00227-5/pdf
  4. Drake N. DNA Test That Distinguishes Identical Twins May Be Used in Court for First Time [Internet]. Wired. 2014 [cited 2017 Oct 10];Available from: https://www.wired.com/2014/12/genetic-test-distinguishes-identical-twins-may-used-court-first-time/
  5. O’Connor A. The Claim: Identical Twins Have Identical Fingerprints [Internet]. The New York Times. 2004 [cited 2017 Oct 10];Available from: http://www.nytimes.com/2004/11/02/health/the-claim-identical-twins-have-identical-fingerprints.html
  6. Humphreys A. New Brunswick twins identical DNA made for difficult conviction in deadly home invasion [Internet]. National Post. 2013 [cited 2017 Oct 12];Available from: http://nationalpost.com/news/canada/twins-convicted
  7. Gilbert M. Police seeing double in rape case involving identical twins [Internet]. CNN. 2004 [cited 2017 Oct 12];Available from: http://www.cnn.com/2004/LAW/06/07/twins/
  8. Guy F. The Criminal Forensic Web of Identical Twins [Internet]. Sword and Scale. 2017 [cited 2017 Oct 24];Available from: http://swordandscale.com/the-criminal-forensic-web-of-identical-twins/
  9. Burke MK. Who’s Your Daddy? Paternity Battle Between Brothers [Internet]. ABC News. 2007 [cited 2017 Oct 10];Available from: http://abcnews.go.com/TheLaw/LegalCenter/daddy-paternity-battle-brothers/story?id=3195632
  10. Ferran L. Rare Twin Murder Case Echoes Bizarre Fingerprint Origins [Internet]. ABC News. 2010 [cited 2017 Oct 12];Available from: http://abcnews.go.com/TheLaw/atlanta-twin-murder-case-echoes-fingerprint-origins/story?id=9909586