Do Identical Twins Share The Same DNA?

We all know that identical twins look the same and may in some cases dress the same, but just how much do they have in common with regards to their DNA?

Before we delve straight in to work through the merits of such a claim, we first give you the differences between monozygotic and dizygotic twins - as they are not the same thing!  

Monozygotic Twins

The correct medical name for identical twins is monozygotic twins, the word monozygotic when translated literally means ‘from one egg’. A zygote is a cell that is formed by one egg and one sperm, so identical twins are formed when this one zygote splits into two. Depending on how early they form they may or may not share the same placenta and the same amniotic sac.

It is not understood what causes the one zygote to spilt into two and no gene has been identified as causing this to happen, but it remains pretty rare. The odds of having identical twins is about 3 in 1,000. 

Dizygotic Twins

The majority of twins born are not in fact monozygotic but are what is called dizygotic. This is where two eggs are fertilised by two different sperm. They are both implanted into the walls of the uterus at the same time. This difference is important as it explains the difference in the connection with DNA. The more everyday term for dizygotic twins is fraternal twins. 

More is known about the causes of dizygotic twins. They are known to occur in women who have a gene which causes them to hyper ovulate, they are more common in women who are overweight, who are taking fertility drugs and we also know that ethnicity and location can play a role. Women with African ethnicity for example are most likely to have dizygotic twins.

Monozygotic Twins and DNA

Monozygotic twins do share the same DNA due to the fact that they are formed from a single source. In testing, monozygotic twins will return results that are 99.99% the same. By comparison, in DNA profiling the chances that one person’s DNA will match the DNA of somebody else is around one in a billion. For siblings it is around 1 in 10,000, but identical twins will have matching DNA sequences.

Identical twins are also always the same gender, although there are some very rare exceptions. The key point to make however is that identical twins are not alike in every way. This is because environmental factors can influence differences such as physical appearance. Some of these differences can even start in the very early stages of development. One example of this is that Identical twins usually share the same placenta, but one twin may have a better and closer connection to the placenta than the other twin, so may receive a better supply of vitamins and nutrients. This can explain why one twin is born bigger than the other.

Epigenetics and Monozygotic Twins

Epigenetics is the study of the external and environmental factors that turn our genes on or off. So, whilst identical twins have the same DNA, we now know that their gene expression can change. Identical twins are crucial in epigenetics because we know that these twins share the same DNA. So, it is epigenetics that may explain differences between twins. One recent study into epigenetics looked at identical twins and the occurrence of rheumatoid arthritis.

We know that rheumatoid arthritis can be affected by genes but also by environmental factors. Studies in identical twins, one of whom had the disease and the other who didn’t concluded that whilst identical twins can share the same DNA, epigenetics-genes being switched on or off in a particular cell ,can then affect which twin got rheumatoid arthritis and which twin didn’t. Epigenetics can also be affected by factors such as smoking, diet and exercise. The findings of most of this research is that identical twins may share the same genomes but have different epigenomes.

It is also interesting to note that identical twins get more and more dissimilar as they age. If we accept that identical twins will have had a similar upbringing, then this is a further example of epigenetic modifications.

One such genetic modification is called DNA methylation. This is where the chemical structure of the DNA changes which goes on to affect how active certain genes are. This change seems to be caused by diet and other lifestyle choices. So identical twins are born with the same DNA but as they age genetic modifications do increase differences between them.

Can DNA Help Distinguish Between Monozygotic Twins?

This question has been an important one for years particularly in paternity cases and also cases where a crime has been committed. But scientists may have now established a way of dealing with this by looking at each twin’s entire genome as opposed to a strand of DNA.

The entire genome could help differentiate between identical twins. This is because whilst identical twins share the same DNA, each will take on mutations as they develop during the early embryonic development stages. This research is still very much in its infancy but in trials conducted so far it has been possible to distinguish between twins but only when the entire genome is examined.

Do Identical Twins Have The Same Fingerprints?

It is believed that there is a genetic element which determines a person’s fingerprint. Fingerprint patterns generally take the form of one of three pattern categories, but fingerprints are not the same in identical twins. This is due to the developmental environment. Put simply, pressure on the skin during development can produce alterations to the skin on our fingers.

So even a difference in the womb position will alter the formation of the grooves and ridges of the skin on the finger. So, whilst we are clear that there is a genetic element which determines fingerprint patterns pressure on the skin during development can produce alterations.

How Much DNA Does the Child Of An Identical Twin Share With Their Parents Twin?

A child of an identical twin will share the same amount of DNA with the identical twin sibling of their parent. This may sound strange, but this is because the DNA between the child’s parent and sibling is the same.

Dizygotic Twins and DNA

Dizygotic twins by comparison have been formed by two separate eggs and sperm so in DNA terms they are similar to normal siblings in that they share about 50% of the same DNA. Unlike monozygotic twins they can be different genders although interestingly 60% are the same gender. Another interesting fact is that dizygotic twins do tend to run in families and tends to repeat down the maternal line.

Differences Between Identical and Fraternal Twins

So, the difference between identical and fraternal twins is in their DNA similarities and differences. Identical twins have the same DNA, non-identical twins do not have the same DNA.

Twins DNA Testing

There are twin’s DNA tests on the market which can determine whether twins are monozygotic or dizygotic. The way these tests work is to look at both DNA samples and see the level of similarity. Most of these tests rely on saliva swabs. Different test providers examine different numbers of markers so do check this carefully.

Why Would You want To Take A Twin DNA Test?

It is not unusual for doctors at the time of birth to misclassify identical with fraternal twins. This is because not all identical twins are developed in one membrane, so if a placental assessment is not performed or the birth makes this difficult then a twin DNA test may be the only way to determine if the twins are identical or fraternal. There are also cases where for example the separate placentas of fraternal twins fuse together and therefore look as if they are sharing the same placenta.

A further reason to take a test is that there could be medical reasons, for example you may want to know if a health condition will affect both twins. Identical twins can also donate organs, blood and bone marrow to each other if that was required as twins experience almost nil rejection in cases of organ donation. 

Another reason you may want to test is for birth control reasons. As we know that dizygotic twins for example tend to run in families you may want to know what type of twins you have to know the chances of it happening again.

What Happens To the Sample in Twin DNA Testing?

The DNA samples (usually saliva swabs) are looked at to see the specific markers present in repeat sections of the DNA. This will determine whether the twins are monozygotic or dizygotic. 

So, what is clear is that identical twins do share the same DNA profile. But it is maybe the word ‘identical’ that is the misnomer. We now know that changes do occur to monozygotic twins in the uterus and continue throughout the life of the twins. So, whilst they have more genetic similarities than most siblings, they do develop their own genetic identities. As we learn more about DNA and epigenetics, we will hopefully understand this even more.