The identical twin does not have either similar DNA or an epigenetic profile.
Twins are the mystery or we can say “miracle” of nature, identical twins are indeed very special in those cases.
Due to the splitting of an embryo, two separate organism evolves in a womb having a similar genome or DNA at that time.
A single embryo has the same type of cell-DNA or similar type of genomic content in all cells thus when it splits, the same type of DNA may have present in another organism as well.
In the identical twins, a single fertilized egg splits into two- called as monozygotic twins. Because of this reason, identical genes or DNA and sex shared between them.
Contrary, in the dizygotic twins, two separate fertilized eggs develops in a womb. Often known as non-identical twins, the dizygotic twins have different genetic composition, sex and characteristics. Even both doesn’t look similar.
The genes or DNA or a genome of monozygotic twins are similar at early-stage, which apparently starts changing under the influence of different extrinsic factors. After the splitting of the embryo, DNA begins diverging immediately.
Once it separates, the DNA starts changing immediately. Two different replication mechanism starts regulating two different cell population.
Notably, despite having similar genes, not all the DNA of the monozygotic twin are similar. For understanding the fundamental better, we need to understand the structure of the genome.
Our genome is made up of coding and non-coding DNA sequences. The coding DNA sequences are “genes” encodes a specific protein consuming ~3% of total genomic DNA.
While the non-coding DNA can’t encode any protein and consume approximately 97% of the genomic region.
Even though it is disable to construct protein, that doesn’t mean it is nonfunction. The non-coding DNA helps in regulating gene expression.
Repeated DNA sequences, microsatellites, minisatellites, telomeric DNA, methylated DNA and transposons are some of the non-coding types of DNA present in a genome.
Our genome is very sensitive to intrinsic and extrinsic stimuli. Random mutations are continuously occurring in a genome and our DNA repair mechanism fixes it all the time.
By using the mechanism called replication, our DNA is copied.
The replication is a process in which DNA polymerase synthesises new DNA. It is one of the intrinsic factors that favour the fact.
As we know that the replication machinery adds random mutations every 100 million base pairs around. 100 to 200 random mutations occur in a genome after each round of replication.
Therefore the DNA replication machinery starts adding random mutations in both types of the cell population.
See the image you can understand what exactly I want to tell you,
Even if the DNA sequence or a gene sequence is similar, different phenotypes are originated. Furthermore, the level of gene expression is different too for the same gene.
One of the major variations between identical twins originates after the post-translational modifications.
Epigenetic alterations such as methylation, histone modification, histone acetylation and chromatin remodelling change the expression of a gene under the influence of intrinsic and extrinsic factors.
See the image below,
The DNA sequence is similar in both but due to the difference in the methylation pattern, copy number variation leads to change in gene expression.
“Even though a genotype is similar in monozygotic twins, due to epigenetic factors phenotypic variations are observed.” Epigenetic factors regulated gene expression in different cells and different tissues.
“A change in gene expression occur without changing the base sequence of a gene is referred to as epigenetic alteration.”
Now let me ask you one question, “ if one of the identical twins suffers from a complex genetic disorder (let say schizophrenia), is it possible that the other person suffers from the same?
No, not at all.
Because the genome of both reacts differently under the influence of different environmental factors.
As they grew up, they face different harsh and favourable conditions, and according to that their DNA or gene behaves.
Let us take another example, suppose one person has the habit to smoke. Abnormal genetic changes occur in that person and he is at more risk to cancer. On the other side, another person from the twin doesn’t have that mutations or alteration in his genome.
That is why,
“No two persons in the world is similar.”
Another important external factor is the environment.
External factors, such as sunlight, radiation, cold, the requirement of nutrition and stress affect our DNA.
Suppose one person from the identical twin working in the high radiation zone, obviously his DNA and epigenetic profile changes faster. And that changes are not at all observed in another person.
Now let’s take the topic even deeper,
The X chromosome inactivation is must required process during embryo formation. Two X chromosomes are present in a female. Among which one must be inactivated, otherwise, overexpression of genes makes it toxic for a cell.
Now let’s take the example of a female- monozygotic twin.
One X chromosomes are inherited from the father and one from the mother, right! After splitting of an embryo- monozygotic female twins are developed.
But!! The question arises here is that which X chromosome is inactivated?
The cells natural X-inactivation mechanism randomly inactivate one of the two X chromosomes of every cell. Here which X chromosome is inactivated? Maternal or paternal? we don’t know.
The ratio of material to paternal X chromosome inactivation is random and varies between both twins.
Another less-considered factor is behaviour. Scientists believed that some of the human behaviour phenotypes are regulated by genes. Although it is controversial and not accepted widely.
But for example, the “rhythmic finding of nipple” is one of the common behaviours observed in almost all mammals. Thus it might be conserved since evolution.
If it is so, twins behave differently in different conditions and environment that make changes to their DNA, but this is just a concept not scientifically proved.
Related article: Behavioural Genetics class 1: Science behind the behaviour.
NASA had performed an experiment on a twin brother Scott Kelly and Mark Kelly (both are astronauts). Their finding suggests that epigenetic and telomeric DNA alters during a stay in spacecraft. Read this article here: NASA Twins Study, “7% Of Scott DNA Is Not Matching With His Twin Brother Mark”, Is It Possible?
During the DNA fingerprinting or DNA profiling- for identification of individuals, scientists are targeting only some of the polymorphic regions. it is possible that those DNA regions between twins may not have changed.
This is the reason, the DNA profile of identical twins are the same but still, both have different DNA content.
If we sequenced entire DNA or genome of both identical twins non-pathogenic, random sequence variations and polymorphism observed.
Identical twins appear similar, even, have similar genes and DNA profile but does not have identical DNA. Various factors play an important role in making them unique. The environment is one of the important factors having a great influence on once gene expression.