When one twin looks at another, they may feel like they are looking into a mirror. After all, what differentiates them from fraternal twins is that they generally have the same DNA and, therefore, are practically identical. But the truth is that in some cases that of the mirror is literal, since there are a few cases, known as mirror cufflinksin which there are some equal characters, but located right on the opposite side of the body. They can be moles, birthmarks, dental abnormalities and much more.
Its origin is not clear. Scientists think it must be something that happens during embryonic development, but they don’t really know what it is. There are some theories, but none have been confirmed for sure.
Yes, there are many documented cases in the scientific literature. In them you can see which are the most common characters in that mirror image of the mirror twins. Although they are also described some oddities. For example, cases of mirror twins with exactly the same visual acuity figures, but in opposite eyes, have been described.
How are twins different from fraternal twins?
Before talking about mirror twins, it is important to differentiate twins from fraternal twins. Twins are the result of fertilization of two ovules by two sperm. It happens simultaneously, so the two babies are born together. However, their genetic information is different and they are not far from any pair of siblings who have not lived together in the womb.
However, twins are formed when a zygote, resulting from the fertilization of a single egg with a single sperm, is divided into two equals that continue to develop separately. For that reason, they do share DNA.
Mirror twins are those who, despite being identical, have some of the same characters, but just on the opposite side of the body.
It is very common in topics related to moles or birthmarks, but also with the teeth. It may simply be a slightly longer fang, as seen in a chapter of the French series ICA. Or something more accentuated, such as unilateral condylar hyperplasia, in which a significant asymmetry develops in the jaw.
It can also occur in connection with certain syndromes. For example, in 2012 the case of a pair of mirror twins with Kabuki syndrome was described. This is characterized by growth retardation, intellectual disability, microcephaly and facial dysmorphic features. In the case of these two they had cleft lip and palatebut all exactly on the opposite side of the face.
It can even happen with fingers accessories. That is, the two brothers are born with an extra finger, but just on the opposite side of the body. In 2021, such a case was described in two sisters who had an accessory toe, one on the left foot and one on the right.
In a 1993 study it was also described that mirror twins could be psychological opposites. That is, they could have, for example, opposite professional interests or different sexual orientation. However, today this is not as explored, since these are factors in which the environment after birth could greatly influence.
Theories about its origin
It is estimated that the phenomenon of mirror twins can occur in the 25% of twin births. With him it is shown that the twins may not be as identical as they seem. Furthermore, since can be related to some diseasescould also help to understand how they originate.
This makes it very important to study its origin. However, to this day it is not known for sure what gives rise to some siblings turning out to be mirror twins. There are several hypotheses. For example, it is believed that it may be related to epigenetics. This is a discipline that studies changes in the expression of genes that do not affect the DNA sequence.
To understand this better we must remember that our genetic material, unique to each individual yet equal among twins, is like our instruction book. It contains everything necessary for our body to function properly, as well as the orders that indicate how we are. But not all of those instructions get used. If we look at the manual of a washing machine, there are parts that we never get to read. For example, it may tell you what to do if it doesn’t spin. But if you always spin properly, you won’t need to read that part.
The same thing happens with our genes. Our DNA contains information that may not be read. Furthermore, although all cells have the same genetic material, not all read the same genes. What does an eye cell want to use the instructions for how to make insulin? That’s a pancreas thing.
When a gene is used, either because it is necessary in certain circumstances or because we are in a cell that needs it, we speak of gene expression. It turns on or off as needed. But there may be some changes due to the environment that modify the expression of genes, placing certain molecules on them, but without altering the sequence. This affects many levels. For example, it is believed that going through a period of great stress, such as a war, can lead to epigenetic changes that would even be passed on to offspring.
It is not ruled out that epigenetics has a certain relationship with the development of mirror twins. But there are also other hypotheses. For example, the zygote divides a little later than usual in the fertilization process. Thus, as Dr. Leann Poston explained in an article for health linethe division would occur at a time when the genes that determine the left or right side in the embryo have already been activated.
Be that as it may, it is a phenomenon worth exploring further. Meanwhile, one in four sets of twins will still feel like they are looking in the mirror when they meet their siblings. Literally.