1. How Identical and Non-Identical Twins Form Before Birth
The first part of the answer lies before birth. There are two main ways twins are formed. One type occurs when a single fertilized egg splits into two during development. These are called identical twins (or monozygotic twins). Because they start from the same egg, their genetic information is nearly the same, and they are basically the same sex. The other type occurs when two separate eggs are fertilized by two different sperm cells. These are fraternal twins (or dizygotic twins). They share genetic information only to the same degree as ordinary siblings. You can think of them as siblings who happened to be in the womb at the same time. This is why combinations of different sexes can occur[1].
If non-identical twins look different, it is no surprise, just like ordinary siblings can look different. Whether they look alike or not, both types are very normal twins, and each person is an individual. In Japan, about 1 in 100 births are twins. According to the Japan Multiple Birth Support Association, the breakdown is a little over 0.4% for identical twins and about 0.6% for fraternal twins[2]. Why one egg splits into two is not yet fully understood[1].
Here, a mystery remains. Identical twins start from one egg. If their genetic information is nearly the same, they should become two exactly the same people. However, that is not what happens. For example, consider fingerprints.
2. Why Twins With Nearly the Same Genetic Information Are Still Different
Identical twins have nearly the same genetic information. Still, they are two separate people. They have similarities and differences. Where do these differences come from? Let us look at three known facts.
Fingerprints are different. Even for identical twins, fingerprints are not the same. This is because factors other than genetics play a large role in fingerprint formation[1]. Even with the same genetic information, how genes work is influenced by many factors, such as how they develop in the womb and the environment[1].
Differences in chemical marks grow with age. Genes have chemical marks that strengthen or weaken their activity. A 2005 study of a large group of identical twins compared these marks. For young children, there was almost no difference. However, for older people, the amount and location of these marks differed greatly[3]. It seems that even if they are nearly the same at birth, differences accumulate as they live their individual daily lives.
There are slight differences in the genetic information itself. A 2021 study compared the genetic information of identical twins. It showed that mutations formed in the early stages of development differed between twins by an average of 5.2 mutations. In about 15% of pairs, the difference was especially large, with only one twin carrying many mutations [4].
How much is inherited? Researchers have compared the similarities of identical and fraternal twins to understand how much of our resemblance is genetic. This is because identical twins have nearly the same genetic information, while fraternal twins share genetic information like ordinary siblings. Over many years, many twins and their families have cooperated in these studies. A 2015 study summarized about 50 years of twin research. It used data from 17,804 traits, 2,748 papers, and about 14.56 million pairs of twins. The proportion of differences between people that can be explained by genetic differences was an average of 49%. This means about half is genetic, and the other half is due to environment and other factors[5].
For about 70% of traits, the idea that gene effects simply add up was enough to explain how alike twins were; the view that growing up in the same home mattered a lot did not fit those traits. However, 49% is an average of a very large number of traits. It does not mean "49% of your height is genetic." It also does not mean that the environment is not involved[5]. Resemblance and non-resemblance involve both genetics and other factors. Even with the same genetic information, each person is unique. So, what about twins in the animal world? There are animals with completely different ways of being born.
3. Armadillo Quadruplets and Twins Who Exchange Cells
Identical quadruplets every time. The nine-banded armadillo gives birth to quadruplets every time. Moreover, they are always four genetically identical individuals from one egg that split into four. The four embryos share one placenta. It is introduced as the only vertebrate that gives birth to identical quadruplets every time. Why this birth method evolved is considered a mystery[6].
If they are four genetically identical individuals, they should look exactly alike. However, a 1968 report in Science News magazine introduced that these quadruplets, despite having the same genes, do not always look the same. It presented this as a question to the idea that "genes alone determine heredity." Later explanations state that chemical marks regulating gene activity can greatly change appearance even if the genes are the same[7]. The "same but not identical" phenomenon seen in human identical twins is not limited to humans.
Twins who exchange cells. Marmosets almost always give birth to fraternal twins. If they are fraternal, their genetic information should be as separate as ordinary siblings. However, during pregnancy, their blood circulations connect, and they exchange blood and stem cells that become blood and immune cells. As a result, they become "chimeras" with mixed cells for their entire lives[8]. A 2024 explanation notes that in the brain, sibling cells were found mainly in immune cells called microglia[8].
Comparison with human twins. The proportion of identical twins in humans is about 0.4%[1][2]. Armadillos are always identical. Human fraternal twins keep separate genetic information, but marmoset fraternal twins mix their cells. Even for "twins," the formation varies greatly depending on the animal.
4. How You Can Investigate Twins and Genetics Further
By now, you may have several "why?" questions. Here are things you can check yourself or investigate further.
Things you can do today. Look at your own fingerprints with a magnifying glass and compare them with those of your family. Fingerprints are explained as being largely influenced by factors other than genetics[1]. Look for similarities and differences between you and your family. Differences are also part of who you are. The reason for resemblance is not always just genetics[5].
If you want to know more. Look up the meaning of the term "heritability." Check what the number "49%" represents and what it does not represent.
Note-taking tip. Writing down just one word that caught your interest can be a starting point for later research. Words like "epigenetics" or "chimera" are enough.