The Genetic Relationship Between Identical Twins

Identical twins are exactly alike—or are they? After all, they're always the same sex, they look exactly alike, they often dress the same (or at least their parents dress them the same when they're little), and they tend to share certain mannerisms and other features.

A frequently asked question about identical twins is, "Do they have the same DNA?" Read on to learn the answer, and why it matters.

How Identical Twins Are Formed

The medical name for identical twins is monozygotic twins. Mono means one and zygote means a cell that's formed by an egg plus a sperm—so identical twins are created when a single zygote splits into two.

As the two zygotes travel to the uterus, they each continue to divide and grow. Scientists don't know what causes a single zygote to separate into two individual ones. Technically, it's considered a malfunction of the normal development process.

The two separate beings who are created in this way share the same DNA, which stands for deoxyribonucleic acid. DNA is made up of the chromosomes that contain the genetic information that determines everything about us—from the color of our hair and eyes to our athletic ability and personality traits.

Environmental factors can influence a person's physical appearance, for example, 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, temperament, or personality.

Identical or Fraternal?

The odds of having identical twins is about 3 in 1,000 and about a third of all twins are monozygotic. The majority of twins are dizygotic, produced when two separate eggs are fertilized by two different sperm. One way to find out if a pair of twins of the same gender is identical or fraternal (different-sex twins are always fraternal), is through DNA testing.

A sample of DNA is obtained by swabbing the inside of each twin's cheek, then the samples are submitted to a service that, usually for a fee, will analyze and compare the DNA from each individual. 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% similar. 

Getting Away With Murder?

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 used this fact as a plot line. How often is it an issue in real life, however? Could identical twins pull off the perfect crime?

It has happened. 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 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 O. (German law doesn't allow full names to be revealed.) Both men were arrested and charged but ultimately were released when it wasn't possible to identify which one had been involved in the crime.

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.