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Identical Twins Are Not Identical

Studies show that the deeper we look into so-called identical twins, the more differences we find.

There is a common parental misunderstanding that arises when the term “identical twins” is paraded around. Identical twins are known scientifically as “monozygotic twins.” This Scrabble winner of a term refers to the fact that these types of twins arise when a single sperm fertilizes a single egg to form a zygote, and this zygote goes on to split into two masses of cells that give rise to the twins. By contrast, fraternal twins are essentially regular siblings that are born at the same time. One sperm fertilizes one egg and another sperm fertilizes another egg at the same time and bingo: fraternal twins.

It has been reported that most parents of identical twins actually believe their children are fraternal twins because they are not identical in every way. As it turns out, “identical twins” is a misnomer. Monozygotic twins, as they should be called, are different in many ways, including in their DNA as a recent study confirms.

The twin connection

Humans are not the only animals capable of producing monozygotic twins. Armadillos are known for them and they have been reported in dogs, horses and pigs as well, but it is difficult to get a handle on how common they are in these species. Without DNA testing, how do we know if two pups came from the same zygote? Conversely, it’s possible that many animal species can conceive monozygotic twins but that these twins rarely survive birth because they have to share nutrients like food and oxygen through a single placenta which may not be sufficient. Humans have become more adept at it, with three to five in 1,000 live births being made up of monozygotic twins. The odds of having twins go up with advanced age, with the number of previous pregnancies, and with the use of assisted reproductive techniques, among other factors.

Twins have always fascinated humanity and they have been the source of many myths, including that they are gifted with a psychic connection. After all, how else to explain that so many twins can finish each other’s sentences? The answer is quite simple, as long-time friends and partners can attest: shared experiences.

As for the astonishing stories of one twin knowing the other is in danger, these anecdotes tend to have more mundane explanations when we dig into them. The 2009 case of Gemma and Leanne Houghton sure seems paranormal. The two twins were teenagers and Gemma suddenly felt compelled to check on her sister. Turns out the sister was unconscious in the bathtub, having suffered a seizure, and she was brought back to life by Gemma’s first aid. But as the deputy editor for Skeptical Inquirer magazine, Benjamin Radford, pointed out in an article on twins, Leanne had a history of seizures and her sister had been told to keep an eye on her. Knowing that her sister was taking a bath alone and not hearing any splashing of the water, it’s no wonder Gemma went to check on her. No telepathy needed.

But the most persistent myth about so-called identical twins is baked into the name. Monozygotic twins are not identical, and this realization has implications for scientific research.

It’s more complicated than nature versus nurture

We expect monozygotic twins to have identical DNA. After all, they come from a single sperm cell (that has 23 of the father’s chromosomes) and a single egg (that has 23 of the mother’s chromosomes) to produce a cell with 23 pairs of chromosomes that goes on to replicate this DNA and divide, and replicate and divide, until two distinct masses emerge to eventually give rise to the twins. Given that our genome consists of 3 billion letters, comparing the DNA of monozygotic twins to see if it really is identical used to be impossible. Shortcuts had to be employed, like testing for specific regions of the DNA known to vary a lot. However, improvements in technology (and in its affordability) now allow scientists to take a much deeper look, and they are finding out that many monozygotic twins are not identical at the DNA level after all.

When scientists recently read the DNA of 381 twin pairs, they reported that, on average, so-called identical twin pairs differed by 5.2 mutations. This is tiny on the scale of 3 billion letters but it is an average. Thirty-nine of these pairs actually differed by more than 100 mutations, while 38 pairs did not differ at all at the level of their DNA.

Where do these mutations come from? Simply put, many of them arise because the enzyme tasked with copying our DNA makes mistakes, and if these mistakes go uncorrected, they stick around as mutations. The more our DNA gets copied, the more opportunities there are for mistakes. When a mutation arises after the cell mass has split itself into two twins, one twin gets the mutation while the other does not. But even if the mutation arises before the twinning process, it may affect only the cells that will make up one twin and not the other, or it can affect some of the cells of one twin, meaning that the adult twin will have the mutation in some of their cells but not in others, a phenomenon known as mosaicism. But mutations aside, there is another important way in which monozygotic twins differ, and it has to do with one of the hottest fields of research: epigenetics.

If genetics explain that, metaphorically speaking, bakers make bread (or that genes make proteins), epigenetics detail the rules and regulations that dictate when bakers make bread and how much bread they make. There are many ways in which our bodies have evolved to dictate when a particular stretch of our DNA should be active, and while these epigenetic markers are inherited from our parents, they can be reversed and change over time, and that means that monozygotic twins can drift away from one another because their epigenetics change independently of one another. It’s a little bit like building identical factories but having each run by a different managerial team. When you go back ten years later, you will notice subtle (and not-so-subtle) changes.

You may be surprised at how broad some of these changes can be in so-called identical twins. In nearly three quarters of cases of monozygotic twins having an autoimmune disease (conditions like lupus and rheumatoid arthritis), only one of the twins is affected. This is known as “disease discordance,” meaning that one twin has a disease whereas the other does not, and it has been reported for conditions like hemophilia, multiple sclerosis and Duchenne muscular dystrophy. Very rarely, some monozygotic twins even end up with different sex chromosomes.

These observations fit into our deepening knowledge of how our bodies are shaped by what’s inside of us and by what’s affecting us from the outside. Our understanding used to be very simplistic: it’s either nature (our DNA) or it’s nurture (our environment). Scientific studies of twins fit squarely into this paradigm. If you saw monozygotic twins that differed in some way, their unshared environment was to blame. Likewise, if monozygotic twins were more alike in some way than fraternal twins (who only share about half of their DNA), then the difference could be fully accounted for by genetic variation. Twins were used as a knife to cut through the nature/nurture debate, and the research emanating from these studies has been very informative.

But we are now realizing that the situation is more complex. The DNA of monozygotic twins tends not to be 100% identical, and epigenetic and environmental differences further widen the gap between twin pairs. It’s not nature or nurture; it’s a complex interaction between our genes, our environment, and our epigenetic markers that shape who we are and what illnesses befall us. And researchers will have to take this into account moving forward when using twins in studies.

Monozygotic twins may at first glance appear identical, but under the surface very complex dynamics are at play. There’s still no telepathy, but the very real dynamics between genes, their regulators, and the environment are infinitely more complex and much more mesmerizing.

Take-home message:
- The scientific term for identical twins is “monozygotic twins” because these twins come from a single sperm fertilizing a single egg, which is called a zygote
- A recent study looking at the DNA of so-called identical twins revealed that, on average, each pair of identical twins differed by 5.2 mutations in their DNA
- So-called identical twins can also differ in the environment they are exposed to and in the epigenetic marks on their DNA which regulate how active their genes are, which can lead to one twin of a pair developing a disease while the other does not


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