Identical twins are assumed to be exactly alike. They look alike, act alike and sometimes even dress alike. One of the frequently asked questions about identical twins is, "Do they have the same DNA?" In order to understand the answer to this question, it's important to examine what identical twins actually are, and how they form.
What are Identical Twins
Identical -- or monozygotic -- twins form from a single fertilized egg that splits into two. (Mono=one, zygote=initial cell formed by egg+sperm) During sexual intercourse, a single sperm meets a single egg. The cells divide and expand as the zygote travels to the uterus. Occasionally, for reasons unknown, the cell divides into two parts that develop as two distinct individuals, creating monozygotic twins. Reasons for the split have not been scientifically identified and confirmed, so the causes remain a mystery. Technically it's a malfunction of the normal development process. The odds of having identical twins is about 3 in 1,000.
What is DNA?
Because the individuals originate from a single combination of egg and sperm, they share the same DNA. According to About.com's Guide to Biology, DNA is "a type of macromolecule known as a nucleic acid. It is organized into structures called chromosomes and housed within the nucleus of our cells. DNA contains the genetic information necessary for the production of other cell components and for the reproduction of life." The DNA, shaped like a double helix, are organized into chromosomes. Every cell contains forty-six chromosomes. The father's sperm contributes half of their chromosomes (23) while the mother's ova, or egg, contributes the other half (23). Since monozygotic twins originate from the same egg/sperm combination, they have the same DNA.
However, although they share the same genetic characteristics, identical twins are not necessarily exactly alike. Environmental factors can influence physical appearance, and epigenetic factors create differences as twins age. The epigenome describes chemical changes within DNA as an organism ages. These factors explain why identical twins can be different, whether in appearance, temperment, or personality. Although some stereotypes establish an expectation that twins are clones, they are unique individuals, despite their genetic similarities.
Monozygotic twins represent about one-third of all twins. The majority of twins are dizygotic, produced when two separate eggs are fertilized by two different sperm and develop during a single gestation. They share about half of their DNA, the same as any two siblings, and will return distinctive results in a DNA test. They are commonly known as fraternal twins.
Identical Twins and DNA Testing
DNA testing is one way to determine zygosity in twins and identify twin type. A sample of DNA is easily obtained by swabbing the inside of each twin's cheek, then the samples are submitted to a testing service. For a fee of about $150, the DNA is analyzed and compared. A typical DNA test performed on monozygotic twins will return results with 99.99% similarity. However, DNA from non-identical (fraternal or dizygotic) twins will generally be about 50% - 75% similar. For many twins, or families with twins, the only way to know for sure whether the they are identical or fraternal is through DNA testing.
Because identical twins have the same DNA, it is nearly impossible to distinguish between these individuals when analyzing DNA for paternity testing or for evidence of a crime. Many literary mysteries, soap operas, and crime dramas have utilized this as a plot line, and the intriguing concept makes great entertainment. How often is it an issue in real life, however? Could identical twins pull off the perfect crime and avoid conviction?
Some cases have occurred. In January 2009, a set of identical twins were suspected in a German jewelry heist. Three thieves entered a luxury department store by sliding down ropes strung from the the skylights and escaped with jewelry worth more than $6 million. Investigators were able to extract a DNA sample from a drop of sweat found on a discarded latex glove found at the scene and identified two suspects, identical twins Hassan and Abbas were arrested and charged. However, the twins were released because of lack of evidence because it was not possible to identify which of the twins were involved in the crime.
In another case, prosecutors faced a dilemma in the case of identical twin brothers charged with the rape of a nine-year-old girl in Houston in 2005 because the DNA sample could not confirm which brother committed the rape while the other drove the vehicle. Eventually one of the brothers confessed and was convicted.
At this time, the technology does not exist to analyze DNA on the level required to distinguish between samples from identical twins. However, it's hoped that DNA testing will evolve to a level where it can the subtle epigenetic changes that exist in twins. Not only would it be useful to forensic scientists, but it would also help identify genetic significances associated with cancer and other diseases.
Interestingly, a more old-fashioned technique in crime investigation actually can be used to discriminate between twins. Fingerprint evidence can help "point the finger" at criminals, even if they are identical twins. That's because even identical twins have unique fingerprints.