[Bit#47] Identical Twins: Copies of the Genome, Yet Completely Distinct Individuals

1. The Beginning of 100 Percent Heritability, When a Single Egg Splits into Two

Every human being begins life as a single, microscopic cell. The fusion of a sperm and an egg creates the zygote, which serves as the definitive starting point. In typical scenarios, this single zygote undergoes continuous cell division to develop into one individual. However, identical twins embark on an entirely different path. What is the reason behind this divergence? It occurs because the zygote perfectly clones itself and splits into two. This phenomenon generally takes place within 14 days post-fertilization. It is a profound moment where a single egg mysteriously divides into two independent living organisms.

Then, how similar is their genetic makeup? The answer is exactly 100 percent identical. This is because they divide the original blueprint held by the initial zygote precisely in half. Every biological trait, including hair color, eye shape, and even inherited blood types, is perfectly replicated. In the scientific community, this is referred to as 100 percent heritability. Literally, they are perfect biological doppelgangers crafted by nature.

An intriguing question arises here. Is this flawless division determined by the genetic traits of the parents? Surprisingly, it is not. While fraternal twins are heavily influenced by family history, the birth of identical twins is a completely random choice of nature. It is not a genetic phenomenon passed down through generations. Worldwide, the probability of identical twins being born remains constant at approximately 0.3 percent. Regardless of race or region, it is a random miracle that can happen to anyone.

Will these two individuals with 100 percent identical genes lead exactly the same lives moving forward? Where do the differences begin for two human beings who share the exact same starting line? These two lives, split from a single cell, already begin to face different destinies inside their mother’s womb.

2. Different Universes Inside One Cradle, Physical Differences Engraved by the Prenatal Environment

Do identical twins with 100 percent identical genes look absolutely indistinguishable? Not quite. They grow together inside the very confined space of the mother’s uterus. Right inside this cradle, the very first differences between the two children emerge. The internal environment of the uterus is not as uniform as one might think. The precise positions where the two fetuses settle differ minutely. This tiny difference in location triggers a massive butterfly effect. Why is that? It is because the amount of nutrients and oxygen supplied varies depending on where the umbilical cord connects to the uterine wall.

One child may settle in a nutrient-rich area of the uterine wall, while the other might be positioned in a location with a relatively suboptimal nutrient supply. Depending on how they share the placenta, a phenomenon can occur where one child draws more blood flow than the other. Consequently, their birth weights can differ by several hundred grams, or in severe cases, by more than a kilogram. Although they possess the exact same blueprint, their physical conditions differ from the very beginning due to their initial growth environment.

There is an even more fascinating fact. Identical twins have different fingerprints. How can fingerprints differ when their genes are identical? Fingerprints are formed between the 13th and 19th weeks of pregnancy. During this time, the fingertips of the fetus touch the uterine wall or encounter the flow of amniotic fluid, creating microscopic ridges. The convection of amniotic fluid and the movements of the fetus are completely random. Therefore, the pressure and friction applied to the fingers of the two children can never be identical. As a result, a unique fingerprint, the only one of its kind in the world, is engraved onto each individual’s fingertip.

The same principle applies to the location of moles on the body or the pattern of the iris. Microscopic stimuli from the prenatal environment influence the process where skin melanin cells migrate and arrange themselves. Ultimately, identical twins may be genetic carbon copies, but physically, they are distinct individuals who have experienced different environments right from the womb. They face different universes within a single cradle, completing their own unique physical characteristics. How then do these microscopic differences widen after they emerge into the world?

3. The Genetic Switch That Alters Destiny, The Divergence of Constitution Made by Epigenetics

Once they emerge into the world, twins begin to lead distinct lives. One might prefer meat, while the other enjoys vegetables. One might exercise rigorously every day, while the other stays up all night gaming and enduring stress. What happens inside the bodies of these two individuals who share 100 percent identical genes? Surprisingly, as time passes, their genetic blueprints begin to operate differently.

The science that explains this phenomenon is epigenetics. Countless switches are attached to our genes. The DNA sequence itself does not change. However, depending on the environment to which a person is exposed, specific genetic switches are turned on or turned off. Why does this happen? The food we eat, the emotions we feel, and our sleep duration attach microscopic chemical tags to our genes. These tags act as control mechanisms that regulate gene expression.

Actual research results are highly compelling. Newborn identical twins exhibit almost perfectly identical gene expression patterns. However, when examining 50-year-old twins who have lived in different environments for a long time, the locations of their genetic switches are completely different. This explains why one twin develops cancer while the other remains healthy for life, even though they inherited the exact same cancer gene from their parents. It is the direct result of dietary habits and stress management flipping the genetic switches.

Ultimately, genes are not absolute dictators that determine our destiny. Identical twins begin life with the exact same genetic blueprint. However, how that blueprint is interpreted and utilized depends entirely on individual lifestyle choices. Though they were identical copies at birth, they transform into one-of-a-kind original individuals as they age. How much more dramatically can this genetic switch change in an extreme environment beyond Earth?

4. Twins Divided Between Earth and Space, The Power of Environment Proven by the NASA Twin Study

The grand question of genetics finally headed beyond Earth into deep space. NASA organized one of the most unique and massive genetic experiments in history, featuring identical twin astronauts Mark Kelly and Scott Kelly. NASA sent Scott into space to reside aboard the International Space Station for 340 days. Meanwhile, his twin brother Mark remained on Earth. They isolated two individuals with perfectly identical genes into completely contrasting environments. Why? It was to precisely measure how an extreme environment like space affects human genes and the body.

One year later, Scott returned to Earth. Scientists conducted rigorous examinations, and the results astonished the world. Numerous changes had occurred in Scott’s body during his time in space. His spine had elongated, making him 5 centimeters taller. His vision had weakened, and the length of his telomeres, the tips of his chromosomes, became longer than Mark’s on Earth. The most shocking discovery lay elsewhere. Even after Scott returned to Earth, about 7 percent of his genes were operating differently than before. The extreme stress of space radiation and microgravity had permanently altered his genetic switches.

This experiment delivered a massive message to humanity. It proved that no matter how identical the genes you are born with, your biological destiny can be completely reversed depending on the environment you inhabit. Twins who were perfect copies at birth became individuals with completely different genetic phenotypes through the massive environmental gap between space and Earth. Environment rewrites the genetic blueprint this powerfully.

Thus far, we have explored how identical twins, starting from a single egg, evolve into distinct original individuals through microscopic variations in the womb and massive environmental differences across Earth and space. If the lives of two people starting with perfectly identical genes can diverge this much, how are your current lifestyle choices altering your genes at this very moment?