Fraternal Twins vs Identical Twins – How Different are they?

Painting of twins on a wall

The process of birthing a new life is a unique one. But it becomes intriguing when instead of one, two lives come at the same time. These babies are called twins. Now, as it is with biology, there are many types of variations in them too. I have discussed two major types in detail and touched, in brief, some others as well. It is fraternal twins vs identical twins. What is the difference? What are the genetic differences in fraternal twins? Or what are the genetic differences between identical twins? What are mirror twins? All the questions, answered here. 

Fraternal twins vs Identical twins 

Fraternal twins are the ones that are less like twins and more like siblings born together. Biologically, a fraternal twin pregnancy occurs when two different eggs are fertilized. This is also called dizygotic (two zygotes). So think about it this way. You and your sibling (if you have one) were conceived like this, but some years apart. Two eggs were fertilized by two sperms, albeit years apart. In fraternal twins, this happens at the same time. 

As a result, fraternal twins are just like siblings with similar but not identical genetic makeup. This is the first point in fraternal twins vs identical twins. Let’s see what identical twins are. 

What are identical twins

An identical twin is a misnomer as they are not identical, both genetically and physically. But they are called identical because of the extreme similarity they possess. They are way more similar than fraternal twins.

Fraternal twins vs identical twins

Identical twins are conceived when a fertilized egg (when sperm and egg fuse) called zygote breaks into two parts. They are also called monozygotic twins. The cells in the zygote are undifferentiated. This means that each of the cells from the zygote has the capacity to form a new human. How or why this divide occurs, no one knows it. But what we get a person divided into two bodies. I’ll explain why they don’t have an identical genetic makeup later in the article. Identical twins are more likely to develop the same disease, like diabetes and high blood pressure. They are also compatible with organ transplants.

Fraternal twins vs identical twins: The key differences.

The key differences between identical and fraternal twins are:

  • Fraternal twins can be of the different or the same gender. If they are two boys, it’s called fraternal. If they are two girls, it’s called sorority. But in the case of Identical twins, it is almost all time of the same gender. So it’ll always be two boys or two girls. There is one exception which I’ll talk about later. 
  • Fraternal twins are more common and the chances depend upon the community, geographical region and the method of conception (IVF, etc). The chances of having fraternal twins are 6 per 1000 in Japan, 20 per 1000 in some parts of Africa and 9 per 1000 in India and Southeast Asia. Interestingly, the rate of Identical twins is always the same, at 3 per 1000. 
  • The blood type of fraternal twins can be the same or different but it is always the same in the case of identical twins.
  • The chances of a person having fraternal twins run in the family whereas for having identical twin has no familial connection. 
  • Both fraternal and identical twins have different fingerprints.

Quick Fact about Identical twins: The children of two identical twins are not considered as the first cousin, but as half-siblings due to their genetic similarity. 

Sub-division of Fraternal and Identical twins. 

The answer for fraternal twins vs identical twins just doesn’t end with their observable differences. There are many differences found in the development of the twins that also put them under different categories. Here are the three types of development twins can have that makes them different or similar. 

Diochorionic Diamniotic Twins – Diochorionic means two placenta and diamniotic means two amniotic sacs. This means that the twins have their placenta to bring oxygen and nutrients and two separate sacs. Fraternal twins have this development with almost every fraternal twin developing this way. Identical twins can also have this configuration if the zygote is divided earlier, within 4 days of conceiving. 

Monochorionic Diamniotic twins – These types of twins have one placenta and two sacs. This is very rare in fraternal twins and most common in identical twins. Almost 60-70% of all twin pregnancy is of this type. Biologically, when the embryo splits within 4-8 days, they get the same placenta but different amniotic sacs. 

The process of twin reproduction
Image: Pranay Mishra

Monochorionic Monoamniotic twins – These twins are rare. In this twin pregnancy, the twins have one placenta and one amniotic sac. This is also considered to be one of the most dangerous twin pregnancies. This is because of the proximity of both the twins as they lay in one sac. The umbilical cord of one baby can get tangled on the other baby, causing oxygen depletion, cerebral palsy and even death. This occurs when the zygote splits after 9 days of fertilization. 

Conjoined Twins – When the zygote divides after 12 days, the twins are not completely separated at all. This results in conjoined twins. They can be mildly connected and separated by simple operations. They can also be attached in a complex way that can include conjoined heart, liver, etc. This needs a far more intricate and risky operation. These twins are also called Siamese twins. The chances of having these twins are extremely low, at about 1 in 50,000 

Why Identical twins are not identical.

The name identical twins is a misnomer because the twins are not identical. They are extremely similar, but again, not identical. There are a lot of reasons for this, including epigenetic markers, environmental conditioning and development factor. So if you take the genetic makeup of one of the twins, it will be slightly different from the other twin. Similarly, the environmental factors and nutrition causes physical differences in the twins. 

There is another class of twins called the semi-identical twins or sesquizygotic twins. In this class of identical twins, one single egg is divided into two and fertilized by different sperms. This results in twins having the same maternal DNA but different paternal DNA. 

Identical twins Mark and Scott Kelly, both former NASA astronauts.
All identical twins are not exactly identical. Identical twins Mark and Scott Kelly, both former NASA astronauts. Image: Public Domain

So there is no direct way of testing for identical or fraternal twins. You cannot look at two people and say they are twins or not for all the cases. Identical twins sometimes look very different. The biological test for identifying identical twins is called the zygosity test which is a non-painful test that compares the DNA and is very accurate.

What are the chances of a person having a twin, or a triplet?

When it comes to fraternal twins vs identical twins, the chances of a person having identical twins is almost always the same, 3 per 1,000 or .3%. We don’t know what factors contribute to this number but studies have shown that even in animals the percentage is similar (0.3-0.4%). The chances of a person having a fraternal twin vary from place to place and it even has a hereditary influence. 

  • Women who are above 35 have a higher chance of getting fraternal twins due to hyper ovulation.
    If a woman’s family had a fraternal twin, she is more likely to have one. 
  • If a woman has had a previous pregnancy of fraternal twins, it is 4 times more likely to have fraternal twins again.
  • Some drugs like clomiphene also increase the chances of hyper ovulation and hence, twin pregnancy.

Looking at some statistics, the number of fraternal twins in the US has increased by 76% from 1980 to 2009. This is attributed to the rise of In vitro fertilization. The number of fraternal twins in the Yoruba tribe of Africa is also very high, touching 45-50 per 1000 people. Countries like India and Vietnam have the lowest rate. 

The rates for triplets is very low, about 1 per 5,000 and a quadruplet is even lower, about 1 per 100,000. 

Rare twin types 

While fraternal twins vs identical twins encompass most of the cases of twin births, there are a few rare cases of twins that do not result in a couple of healthy babies. These are some of the rare and dangerous twin pregnancies; 

Vanishing twins – In this case, the other baby isn’t developed and it dies out. The dead fetus is then dissolved inside the womb. 

Chimerism twin – This type of twin occurs when a part of the other fetus is attached to the living baby. It results in a healthy baby having an extra arm or leg or another arm. 

Parasitic twins – As the name indicates, one of the twins takes all the nutrients and oxygen supply, killing the other one. 

Two conjoined twins from thailand
Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term “Siamese twins”. Image: Public Domain

Are Twins different from Other people?

There isn’t any noted difference between normal singletons. However, it was found that the gestation period of twins, when compared to singletons (single delivery), was less. The gestation period for twins was 37 weeks as opposed to 39 weeks of a singleton.

It was also observed that twins developed language ability later when compared to singletons. This was due to these twins talking in their own made-up language. 

Reference – Hall, J.G. (2003). Twinning. The Lancet, 362(9385), 735-743. DOI: 10.1016/S0140-6736(03)14237-7.

McNamara, H.C., Kane, S.C., Craig, J.M., Short, R.V., & Umstad, M.P. (2016) A review of the mechanisms and evidence for typical and atypical twinning. American Journal of Obstetrics & Gynecology, 214(2), 172-91. DOI: 10.1016/j.ajog.2015.10.930.

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