Identical vs fraternal twins

As a leading provider of DNA testing, we are often asked about the science behind DNA: especially when it comes to twins.

Although thousands of babies are born in Ireland every year, only around 2 per cent of pregnancies result in twins.  

In this article, we discuss the difference between identical and non-identical (fraternal) twins, how different types of twins are formed in the womb, and how twin DNA (zygosity) testing can help families find out whether twins are identical or non-identical.

What’s the difference between identical and fraternal twins?

Whether twins are identical or fraternal depends on how the babies are formed in the womb.

Identical twins are known as monozygotic. This means that they are formed from a single fertilised egg which went on to split into two separate embryos.

As such, identical twins have DNA that is the same (barring minor variations that are very difficult to detect), meaning that they usually have similar features and can be more difficult to tell apart.

Fraternal twins or non-identical twins are known as dizygotic. These twins are the result of two separate eggs, fertilised by two separate sperm.  

As such, these siblings are no more alike than siblings from different births, and may be both male, both female, or one male and one female.

Fraternal twins share DNA in common, as in the case of siblings from different births and may grow at different rates, while identical twins will usually follow the same pattern of growth.

What are identical twins?

Identical twins are also known as monozygotic twins and are the result of a single fertilised egg which splits into two after conception. 

Identical twins share the same genetic profile including the same sex and blood type. They will also have the same physical characteristics and growth pattern.

However, there are some differences: each twin has different fingerprints and teeth marks, and will often have different-handedness and hair whorls.

How are identical twins formed?

Identical twins are formed when a single sperm fertilises an egg (zygote) which then goes on to split into two.

The result is two embryos with near identical genetic codes. This formation of monozygotic twins usually happens during the earliest stage of foetal development when the fertilised egg is merely a cluster of cells. The twins may or may not share a placenta.

When the egg splits in two determines whether the twins will share a placenta. The earlier the egg splits, the more independent an embryo will become by having its own amniotic sac, chorion (outer membrane) and placenta.

There are three medical definitions for the way in which identical twins are formed:

  • Dichorionic diamniotic (DCDA) twins
    Each twin has their own separate placenta with its own separate inner membrane (amnion) and outer membrane (chorion). One-third of identical twins are DCDA twins.
  • Monochorionic diamniotic (MCDA) twins
    Each twin shares a single placenta with a single outer membrane and two inner membranes. This usually occurs when a single fertilised egg splits after 4–8 days and accounts for 70 per cent of all identical twin babies.
  •  Monochorionic monoamniotic (MCMA) twins
    Both share the inner and outer membranes. This usually occurs when a single fertilised egg splits after 9 days of fertilisation.

MCMA twins are easy to identify on an ultrasound scan, and pregnant women carrying MCMA twins are usually more closely monitored throughout pregnancy, due to an increased risk of twin-to-twin transfusion syndrome (TTTS).

Only 1 per cent of identical twins are MCMA.

Do identical twins have the same DNA?

Identical twins share the same genetic makeup and therefore share the same DNA.

Research shows that identical twins have near-identical DNA profiles, while fraternal twins only have 50–70 per cent DNA similarities in common with one another, as is the case for siblings from different births.

These genetic similarities explain why identical twins look alike, and why they usually have the same characteristics, abilities, and behaviours.

However, even twins who look identical and share the same DNA have unique fingerprints.

Identical twins start out with the same fingerprints when they are conceived, but as the foetuses develop in the womb, they are affected differently by small changes in the womb environment, such as fluctuations of hormone levels.

In addition, as the twins start to move and touch the amniotic sac, unique ridges and lines are formed that result in different fingerprints. This is thought to occur from weeks 6–13 of the pregnancy.

In the same way, identical twins also have different patterns of moles and freckles. This is caused by random mutations and will vary between identical twins.

Do identical twins have the same blood type?

Yes, identical twins have the same blood type. This is because they share the same genetic profile.

How common are identical twins?

Monozygotic (identical) twins do not run in families. However, if you’re trying to conceive, you have a 1 in 250 chance of conceiving identical twins, with the odds being the same for everyone.

If you are hoping to conceive non-identical twins, there are a few factors that can increase your chances including:

  • If you’re over the age of 35, you have an increased chance of conceiving twins, because you are more likely to release more than one egg during ovulation
  • If your mother conceived non-identical twins, you may have a higher chance of conceiving twins. This is because the tendency to release more than one egg during ovulation can be inherited from your mother
  • Non-identical twins are more common in certain ethnic groups, with Africa having the highest rate of twin births

What are fraternal twins?

Fraternal (dizygotic) twins are non-identical twins and are the result of two eggs being fertilised by two separate sperm, resulting in two zygotes (fertilised eggs).

As such, these siblings are no more alike than siblings from different births. However, fraternal twins will still share some DNA in common.

Fraternal (non-identical) twins can be conceived at different times, and, in extremely rare cases, they can even have different fathers.

This occurs when twins are conceived from two eggs released at the same time during ovulation and fertilised by sperm from different men (heteropaternal superfecundation), or when an egg is released and fertilised while the mother is already pregnant (heteropaternal superfetation).

How are fraternal twins formed?

Fraternal or non-identical (dizygotic) twins occur when two separate eggs are fertilised by two separate sperm, resulting in two zygotes (fertilised eggs).

All fraternal (non-identical twins are dichorionic diamniotic (DCDA) twins. This means each twin has its own separate placenta with its own separate inner membrane (amnion) and outer membrane (chorion).

Are fraternal twins hereditary?

Yes, heredity on the mother’s side increases the odds of having fraternal twins.

Research has found that having fraternal twins in a mother’s immediate family may double the chances of conceiving non-identical twins. This is because a certain gene predisposes some women to hyper ovulation, where more than one egg is released during each menstrual cycle.

If a man inherits the hyper ovulation gene from his mother, he may pass this gene on to his daughter. His daughter, in turn, is then more likely to release more than one egg when she ovulates and therefore could conceive fraternal twins.

What are your chances of having identical or fraternal twins?

According to the Health Service Executive (HSE), around 2 per cent of births in Ireland are twins.

Globally, twinning rates have increased from 9 to 12 per 1,000 births since the 1980s, with the greatest rises seen in North America, Europe and Asia.

Fraternal twins are more common: with two-thirds of all twins being fraternal/non-identical/dizygotic, and one-third being identical/monozygotic.

Most variations in twinning worldwide seem due to birth rates of fraternal twins. If a mother has had fraternal twins before, she stands a higher chance of having them once more.

The chances of having non-identical twins naturally also increases with age, peaking in women aged 35-39.

The increase in the number of twin births globally has also been driven by ease of access to fertility treatments such as in vitro fertilisation (IVF).

Can non-identical (fraternal) twins have different dads?

Although extremely rare, it is possible for non-identical or fraternal twins to have different biological fathers.

There are two ways in which non-identical twins can have different dads:

  • Heteropaternal superfecundation
    This occurs when twins are conceived from two eggs released during the same ovulation and fertilised by sperm from different men. 

    Although the egg is only viable for 24 hours, sperm can survive for up to six days inside the uterus.

    Heteropaternal superfecundation can only occur if a woman has released multiple eggs and has sex with different men during the fertile window (when the egg is viable). 

  • Heteropaternal superfetation
    Although this also results in non-identical twins, it occurs in a slightly different way to superfecundation. 

    Heteropaternal superfetation is when a second pregnancy begins while the woman is already pregnant. While a rare occurrence, it is possible for ovulation to occur during the first trimester of pregnancy, making a second pregnancy possible if the egg is fertilised during the fertile window.

    While common in animals such as rabbits and fish, superfetation is less common in humans due to the formation of the mucus plug at the entrance of the cervix when a woman becomes pregnant.

    This mucus plug makes it difficult for sperm to make it into the cervix to fertilise the egg. The presence of the first embryo can also make it difficult for the second embryo to implant in the uterus.

What are the odds of fraternal twins being the same sex?

During a fraternal twin pregnancy, there is a 25 per cent chance that both babies will be boys, a 25 per cent chance that both will be girls, and a 50 per cent chance of one baby being a boy, and one being a girl.

Although fraternal (non-identical) twins share some DNA, unlike identical twins, they do not share the same genetic profile.

This means that if one of the twins is a boy and the other is a girl, it is impossible for them to be identical twins. They can only be fraternal or non-identical twins.

Women have XX chromosomes, while men have XY chromosomes. When the fraternal twins are a boy and a girl, this means that one egg was fertilised by sperm carrying an X chromosome, and the second egg was fertilised by sperm carrying a Y chromosome.

What do fraternal twins look like?

Fraternal twins are no more alike than siblings from different births, and may be both male, both female, or one of each.

Some fraternal twins will appear different immediately after birth and can be identified separately according to their size, facial characteristics, and weight. However, some may be alike to the extent that their parents cannot tell them apart.

As they grow, each fraternal twin will have different characteristics.

Do fraternal twins have the same DNA?

Fraternal twins share DNA in common, much like siblings from different births.

However, unlike identical (monozygotic) twins, they do not have the same genetic profile.

Do fraternal twins have the same blood type?

Fraternal twins may have the same blood type, or they may have different blood types. They are no more genetically alike than siblings from different births.

How can I find out if twins are identical?

If you are pregnant and expecting twins, medical professionals will attempt to identify whether your babies are identical or non-identical, via ultrasound scans.

While in the womb, babies are surrounded by two membranes: an inner membrane (amnion) and an outer membrane (chorion).

By examining these membranes using an ultrasound scan, it is sometimes possible to determine whether twins are identical or not, meaning that your medical care can be tailored to suit your needs and the needs of your twins. However, it is not always possible to tell whether twins are identical or non-identical during pregnancy.

By the time twins are around two years old, whether they are identical or fraternal (non-identical) will often become apparent from their physical features.

For example, hair and eye colour, ear shape, teeth formation, and the appearance of hands and feet can indicate whether twins are identical. If twins do not look much alike at all, there’s a strong chance that they are non-identical (fraternal).

While identical twins share their DNA, and will usually look extremely alike, it may not be as obvious in some cases. In some instances, there can be minor differences between siblings that lead their parents to believe they are non-identical.

The only 100 per cent accurate and reliable way to confirm whether twins are identical or non-identical is to take a zygosity DNA test (also known as a twin DNA test).

The test works by analysing DNA samples from multiple siblings to determine whether they are identical or not.

What is a twin DNA/zygosity test?

A twin DNA test – also known as a zygosity DNA test – is a DNA test that can be used to determine whether multiple children from the same birth (twins, triplets, quadruplets etc.) are identical (monozygotic) or non-identical (fraternal or dizygotic).

It is also the only conclusive way of determining whether siblings are identical or not.

While medical professionals will attempt to establish whether twins are identical or non-identical in utero (in a woman’s uterus), it is not always possible.

This is because the placentas of fraternal twins can fuse together, giving the appearance of a single placenta, and giving the impression that they are identical siblings. Similarly, approximately one third of identical twins have separate placentas making them appear fraternal.

How is a twin DNA test performed?

An AlphaBiolabs Twin DNA Test (zygosity test) is performed using the very latest technology and techniques in DNA analysis, to determine whether siblings from multiple births are identical or non-identical.

The test requires a cheek (buccal) swab to be rubbed quickly and painlessly on the inside of each sibling’s cheek to collect cheek cell DNA. This simple, non-invasive method of sample collection can be done at home.

Once the DNA samples arrive at the laboratory, expert geneticists compare both samples, analysing up to 42 DNA markers (loci) to identify matching DNA markers.

If the siblings are identical, they will share the same DNA profile. If they are non-identical (dizygotic or fraternal twins), they will have different DNA profiles, but they will still share DNA in common, like siblings from different births.

Once the Twin DNA Test is completed, a zygosity certificate is issued confirming the results. This can make a unique gift for multiple birth parents, who want to confirm whether their children are identical or non-identical.  

How soon can I do a twin DNA test?

A twin DNA test or zygosity test can be performed on a person of any age, from newborn babies to grown-up children and adults.

As with all forms of DNA testing, consent must be provided by or on behalf of all sample donors taking part in the test.

Any person aged 16 years or over must provide their own consent (a signature) for their DNA samples to be tested.

For children under 16 years of age, consent must be provided by a person with guardianship for the chid(ren).

Why should you find out if your twins are identical or non-identical (fraternal)?

There are many reasons why you might want to order a Twin DNA Test.

Examples include:

  • It wasn’t clear during pregnancy (in utero) whether the twins were identical or non-identical. Identical twins do not always share the same placenta and sac – it all depends on when the egg splits
  • You want to assess the likelihood of conceiving twins again
  • One sibling has been affected by an illness, and you want to know whether you need to test the other sibling for the same condition
  • You need the information to take part in a study about multiple births
  • You want to buy a Twin DNA Test as a gift for multiple birth parents or their children

Knowing whether twins are identical or non-identical can also be useful for reinforcing the need to treat siblings as individuals.

Are blood samples needed for a twin DNA/zygosity test?

No blood samples are needed for a twin DNA test.

The test involves using a cheek (buccal) swab to collect cheek cell DNA from each sibling taking part in the test. The test is non-invasive, with no needles required.

When will I receive the results of my twin DNA/zygosity test?

For just €139, you will receive the results of your twin DNA test the next working day. * Alternatively, for an additional €50, you can choose to receive your results the same working day. *

All DNA test results are emailed out from 4:30pm each day.

*working days from when the samples arrive at our laboratory before 10am

Where can I buy a twin DNA test?

It’s easy to order an AlphaBiolabs Twin DNA Test online now, for just €139.

Your test kit will contain everything you need to collect the DNA samples at home and return them to our laboratory.

Your secure, password-protected results will be emailed to you the next working day (included in the cost of your test). Alternatively, you can choose to receive your results the same working day, for an extra €50.

It’s important to remember that consent is needed from anyone submitting their DNA samples for testing. Anyone aged 16 or over must provide their own consent (a signature) before the test can take place.

When testing children under 16 years of age, consent must be provided by a person with guardianship for the child(ren).

Need more information on our Twin DNA Test? Call our Customer Services team on 01 402 9466 or email info@alphabiolabs.ie.

Twin DNA Testing

Buy Ireland’s fastest Twin DNA Test online now for just €139.

Casey Randall AlphaBiolabs

Casey Randall

Head of DNA Testing at AlphaBiolabs

Casey joined the AlphaBiolabs team in 2012 and heads up the DNA laboratory.

An expert in DNA analysis and a member of the International Society for Forensic Genetics (ISFG), Casey holds an MSc with Distinction in DNA Profiling and a First-Class BSc with Honours in Forensic Science.

Casey is responsible for maintaining the highest quality testing standards, as well as looking for ways to further enhance the service that AlphaBiolabs provides and exploring new and innovative techniques in DNA analysis.