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Do Identical Twins Have the Same DNA?

This may seem like a simple yes/no question… and the simple answer is ‘yes’. However, in reality it’s more complicated than that, and I’ll do my best to simply explain why. Identical twins, unlike fraternal twins, are natural clones of a single fertilized egg. No-one really knows why it happens, although I delved into some of theories a little while back with ‘Identical Twins: Why does the fertilized egg split?’. Cloning, in theory, creates an exact copy of a genetic blueprint; but is that always the case? Do identical twins have the same DNA? 

do identical twins have the same dna

Crime Case Study:

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

“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,” (Prosecuter David Deacon)

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. Those are pretty strong odds.

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 learnt a little about the building blocks of DNA and cell division. As as 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.
do identical twins have the same DNA? helix diagram
source: getrevising.co.uk

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. 

do identical twins have the same DNA? sequencing image
Credit: Gerald Barber, Virginia Tech. Source: nist.gov

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.”

do identical twins have the same DNA compare diagram
Source: answersonlin.blogspot.com

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. 

While the sequence of DNA may not be affected by your environment, the way genes work—called gene expression—can. 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; and 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. 

Despite the majority of differences between identical twins being due to environmental factors, very occasionally there comes a case of identical twins born looking a little different due to a genetic mutation. The majority of somatic mutations occur in cells you would never see or notice. But these identical twins born in the UK in 2015 shocked doctors by expressing different eye and skin color. Amelia and Jasmine 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.

do identical twins have the same DNA? monozygotic twins looks different
source: dailymail.co.uk

So… Do Identical Twins Have the Same 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 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.

twin-pickle-signature

 


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References:

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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