Do Identical Twins Have the Same DNA?

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Identical twins are broadly described as having the same DNA. They are naturally occurring clones, created from a single fertilized egg which splits to create two copies. In truth, identical twin genetics is more complicated, because nature has its imperfections. This article delves into the details, exploring the true answer to “Do identical twins have the same DNA?”.

No-one knows for certain why the fertilized egg splits to create identical (monozygotic) twins. No-one even fully understands how mutations occur. What we do know is without naturally occurring genetic mutations, the theory of evolution falls apart. As science has progressed we are able to detect the smallest of differences between DNA as it mutates, and this gives us more insight into the differences between identical twins.

do identical twins have the same DNA title pin

Identical twins never cease to impress when it comes to science and mind-spinning stories of mis-identity. Take a look at these criminal cases complicated by identical twin suspects:

A Criminal Case Complicated By Identical Twins

In 2004, DNA testing brought to trial Dwayne McNair for the assault, abduction, and rape of two women in Dorchester, UK. It turned out, Dwayne had an identical twin brother, Dwight McNair. The case fell apart because the DNA examined from the rapist’s semen could not pinpoint which twin was to blame. 

“Ordinary DNA science, the kind of science that’s used around the world every day in courts to identify people, can’t differentiate between identical twins,” Deakin [case prosecutor] said.

Boeri, D. 2017. Standard DNA Testing Can’t Differentiate Between Identical Twins. A New Test Challenges That.

However, it was only a matter of time before science caught up with this genetic loop hole. In 2014, prosecutor David Deacon stumbled across ‘massively parallel sequencing’, the next generation in DNA mapping, successfully tested on identical twins in Germany. By comparing the DNA of the rapist’s semen with the saliva of Dwayne, and his brother Dwight, this new technology shows that Dwayne is 2 billion more times likely to be the rapist than his brother.

This is the first time ‘massively parallel sequencing’ has been admitted as evidence in a courtroom anywhere in the world. The defense are trying to dismiss the evidence, mainly on the basis that the lab is comparing semen DNA with saliva DNA. The case is ongoing.

What is DNA?

Depending on when you went to school, you probably learned a little about the building blocks of DNA and cell division. A speedy recap:

  • Our cells carry 46 chromosomes (23 from each of our parents) in the cell nucleus;
  • A chromosome is made up of one long strand of DNA;
  • A gene is a section of DNA which corresponds with a particular characteristic;
  • A DNA strand is made up of six smaller molecules – deoxyribose, phosphate, and four different nitrogenous bases (adenine, thymine, cytosine, and guanine);
  • These four bases are held together in pairs inside the DNA. There are around 25,000 pairs of bases in every human strand of DNA.
  • The order and combination of these base pairs is a unique blueprint for the human body.
DNA structure and bases color FR

What is DNA Sequencing?

DNA sequencing maps out the pattern of bases inside the DNA. And DNA is very small, so this is a difficult and laborious task:

“The diameter of a DNA molecule is about 2 nanometers. A nanometer is one billionth of a meter. A human hair is 100,000 nanometers across.”

Julie Newton, National Human Genome Research Institute

DNA sequencing has been slowly improving since it’s development in the 1970s, but it was not created purely to bust the occasional twin criminal. Understanding the natural process of DNA cloning in cell growth is crucial in understanding how mutations occur, leading to diseases such as cancer or heart disease. Sequencing technologies are also busy developing cures for known genetic conditions such as muscular dystrophy, cystic fibrosis, and Huntington’s disease. 

DNA sequence

The sequencing process starts by sorting pieces of DNA into different lengths by using electrodes to make the strands move through a gel. Different lengths move at different speeds, thus separating them. Florescent dyes are then added to copied batches of the different lengths of DNA, and dependent on which dye is detected by the computer, each strand can be identified with a particular base at one end. By identifying many ‘ends’ of smaller pieces of DNA, the whole strand can be slowly mapped together. It’s complex, slow and expensive, but the process is improving all the time.

Why Would Identical Twins Have Different DNA?

As I said earlier, the simple answer to ‘Do identical twins have the same DNA?’ is yes. As my doctor rather sweetly put it:

“Identical twins happen when a baby likes itself so much it makes a copy.”

Identical twins lg

In truth, scientists don’t call the ball of cells that splits into two ‘a baby’, but it’s certainly on the way. At this point the cells are all the same, no specialized brain, heart or skin cells. So when the ball of cells divides you really do have an equal, identical copy of the original fertilized egg. But next generation DNA sequencing of identical twins now shows slight differences. It seems no-one’s perfect, and there are occasional copy errors in the cloning process, whether it be early on as the cells are becoming a baby, or whether it’s later as we continually produce new cells to grow. These ‘somatic mutations’ can occur at any point, and nearly all of them would not be physically noticeable… tiny differences in the code that only advanced sequencing can detect.

Somatic mutations we are more familiar with are those that cause cancerous tumors, because this particular mutation causes the cells to copy in overdrive, showing themselves dramatically in the form of a tumor. 

Differences between Identical Twins

Nearly all the physical and personality differences seen between identical twins are not due to cell mutations. Environmental factors such as diet, sleep patterns, exercise and exposure to toxins have ‘epigenetic effects’ on the body, which changes how genes express themselves. 

These structural changes can result in slight changes in gene activity; they also can produce more dramatic changes by switching genes on when they should be off or vice versa.

Duke Magazine, Can Your Environment Change Your DNA?

Research in 2004 looked at the development of Alzheimer’s in identical twins. Only 40 percent of the time did both twins develop the condition by their late 70s; in many cases, one did and one didn’t. This is unlikely due to cell mutations, and more likely due to lifestyle differences which have ‘switched on’ or ‘off’ particular parts of their DNA. Both twins start with the same genetic likelihood of developing Alzheimer’s, however, whether particular genes are expressed will depend on the environment their body lives within. Not only can epigenetic effects change how identical twins look and behave, these structural changes in gene activity can also be passed on to future generations. 

Cloned mice with different DNA methylation

 Geneticist Emma Whitelaw studied epigenetic phenomena in mice and observed a genetic mutation in the tails of cloned mice. She also noted more subtle variations in the coat color gene. You could come up with all sorts of theories as to how genes could be impacted by their lab environment but when nature does it without help, things get really interesting…

Very occasionally there comes a case of identical twins being born looking notably different due to a genetic mutation. The majority of somatic mutations occur in cells you would never see or notice. But Ameilia and Jasmine, identical twins born in the UK in 2015, shocked doctors by expressing different eye and skin color. The girls are monozygotic twins, meaning they came from a single fertilized egg. Yet, somewhere along the journey, something happened to the gene in charge of pigment. This altered DNA was copied again and again until the girls were born, and now they look strikingly different because if it. Check out their story and picture –> here.

Conclusion: Do Identical Twins Have Identical DNA?

In essence, yes. Identical twins share the same genetic code because they started with the same recipe. However, if you look closely enough, you’ll find some tiny differences in the sequence due to occasional spontaneous mutations that happen during cell division. In addition, epigenetic effects due to the environment can change how genes are expressed. Once a mutation or structural change has occurred, it will be copied again and again, making it potentially detectable throughout the body, depending on how early it occurred. So to be accurate… no, identical twins do not have [exactly] the same DNA.

Mind blown? I hope so.

 Related Posts:


Boeri, D. WBUR News Standard. (2017, March). DNA Testing Can’t Differentiate Between Identical Twins. A New Test Challenges That

Britt, R. Live Science. (2005, July) Identical Twins Not So Identical

Duke Magazine. (2012, August). Big Question: Can your Environment Change Your DNA? 

Genome News Network. How Does DNA Sequencing Work?

Martincorena, I. Campbell, P. Science Magazine (2015, September). Somatic mutation in cancer and normal cells

National Human Genome Research Institute. (2015, December) What is Genetic Sequencing?  

Terrell, M. Quick, B. CNBC (2016, May) Editing our genes to cure — not just treat — disease

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About the author

Katherine is an electrochemist, hiking enthusiast, and family lifestyle blogger. As a mom of three, including twins, she enjoys DIY, travel, and eating good food. British born, Katherine moved to the US in 2014, and now called Las Vegas home.

11 thoughts on “Do Identical Twins Have the Same DNA?”

  1. Wow, you have properly taken me to school! What a well-researched and informative post. To be honest, I’d never even really thought about it. But now I have something interesting to say on the topic should it arise at dinner parties (should I ever have any). Thank you for linking up to #EatSleepBlogRT. Hope you can come back next week.

  2. What a fascinating read! I would have answered yes, but it’s really interesting how there can be tiny variations even in identical twins. And the identical twins with the different eye and skin colours is amazing! Thanks for joining us at #SharingtheBlogLove

  3. This was really interesting. I would have thought yes, because thats why they are identical. But I didn’t realise everything that went into it and what little things could have a slight variation. Thank you for joining us at #SharingtheBlogLove


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