How twins happen: Fraternal vs Identical

Twins, good for minor characters in creepy horror films and known for the special bond that they’re supposed share. But what exactly is the difference between identical and non-identical twins I hear you say. Well, let’s get down to the science.

Copyright © 1980 Warner Bros. Entertainment Inc.

If you need a description of the first stage required, this is not the place for you. However, if you do yearn for further information, I must direct you to a parent/trusted parent figure. If one is not available, I don’t suggest googling it.

Half of your genetic material comes from each parent, whether it be from a sperm cell from your father, or an egg from your mother. Both of these cells are called gametes and have 23 of the total 46 chromosomes. The chromosomes contain all your DNA which in turn encodes all the genes in your body.

In total, 22 of these chromosomes are termed autosomes, and are paired with 22 chromosomes from the other parent, and final pair are the sex chromosomes. These can either be an X or a Y, with the Y being exclusively available from your father and will determine whether the offspring with be female or male, respectively. The mix of the genes from both of your parents is the reason you look a bit like each of them. If, however, you look like only one of your parents, you may want to give the milkman a critical look next time there’s a delivery.


Half from each of your parents, one of whom may well be the milkman.
Andreas Bolzer, Gregor Kreth, Irina Solovei, Daniela Koehler, Kaan Saracoglu, Christine Fauth, Stefan Müller, Roland Eils, Christoph Cremer, Michael R. Speicher, Thomas Cremer, PLoSBiol3.5.Fig7ChromosomesAluFishCC BY 2.5

The sperm that ends up fertilizing the egg is one of somewhere in the ball park (ahem) of 200 to 500 million sperm cells. The winner of this first race results that reaches the egg and burrows through the outer layer, fertilizes it, forming a zygote cell that is the very first cell. This is the first time all the genetic material which makes you, you, is in the same place at the same time. From this single cell, all subsequent cells are formed. Every cell in your body can trace its lineage from this single instance.

This is where the difference between identical and non-identical, or fraternal twins lies.
Fraternal twins arise from the fertilization of two different eggs at the same time and do not share genetic material any more than offspring who are years apart. They may be born at the same time, but they might be different genders, and will look as similar as your normal everyday siblings.

The prevalence of fraternal twins varies greatly around the world and the title of the twin capital is highly disputed. However, the highest levels of fraternal twins are found in central Africa, with Benin leading the way at 27.9 fraternal twins per 1000 births compared to the global average of 13.1. There are also several other factors that can increase the probability of twins, such as in vitro fertilization, the woman being over 30 years old and being greater than average height and weight.

Identical twins are where things get interesting. They develop from one fertilized embryo that splits to form two zygotes with identical genetic material. Interestingly, the mechanisms that causes the splitting largely remains a mystery. The occurrence of identical twins is believed be at a relatively constant level throughout the world, with somewhere between 3.5 and 4 births per 1000 producing identical twins.
However, despite being identical twins, people that know them well are easily able to tell them apart thanks to some minuscule differences.

Interestingly, identical twins also have different fingerprints, as these are most likely formed by the mechanical movement of the fetus in the womb during pregnancy. Many of these differences arise thanks to epigenetic factors that are influenced by the environment. One of the major processes involved in epigenetic modulation is the methylation of your DNA. These changes alter the expression and use of the target genes and their respective proteins. All sorts of life events can affect DNA methylation such as diet, illness and aging, as well as exposure to chemicals in the environment such as smoking, drugs and medication.

Identical twins
In the immortal words of Baz Luhrmann, wear sunscreen.
St2671, Twins – Gay and Gwyn, Sun exposure, Cropped by No, CC BY-SA 4.0

A relatively new area of study in genetics is the study of genetic imprinting, which is the inheritance of epigenetic changes from your parents. These changes have been found to last for as many as two or three generation, with children and grandchildren of individuals who endured starvation during WW2 still being smaller than average in height.

The studying of twins has also shed light on the magnitude by which the environment can influence an individual. For instance, only about 50% of twins develop Alzheimer’s disease, suggesting that while there is a strong genetic factor, the environment also plays an important role.

So regardless of your position in the twins – creepy or not argument, you can sleep safe in the knowledge that no matter how similar they may look in your nightmares, they’re never exactly the same. However, it still goes without saying that there’s still a special place in hell for people that dress them the same.

Nope, not scared at all…CC image courtesy of ThatMakesThree

A bit thanks to DIYing Machine and Paminat0r for the idea for this post. Be sure to check out their stuff and spread the love!

Any questions that have have you vexed? Never figured out how something in biology works? Drop me a line and I’ll see if I can set that right.


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