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Identical or Non-Identical? Understanding Zygosity Testing

by | Jun 24, 2025 | Zygosity

The unique bond between twins, triplets and higher-order multiples naturally leads to the question: “Are they identical or fraternal?” While it can sometimes be clear from appearance, zygosity testing is the most reliable way to determine whether they share the same genetic profile. Learning how zygosity testing works and what it reveals can help families understand and celebrate everything that makes being a multiple birth family so special.

What is zygosity testing?

Zygosity testing is used to determine if multiple children, such as twins or triplets, are genetically identical or not.

  • Identical siblings are also known as monozygotic. This means that they developed from a single fertilised egg that split into separate embryos. These siblings are always the same sex and share 100% of their DNA.
  • Non-identical siblings can be referred to as dizygotic or fraternal. They developed from separate eggs that were fertilised by separate sperm. Genetically, this means that they share about 50% of their DNA in common – just like siblings from different births – and may be the same or different sexes.

For triplets or higher-order multiples, zygosity testing can reveal whether all are genetically identical, or if there is a mix, such as two being identical and the third being fraternal.

Why is zygosity testing done?

Testing is carried out for a range of reasons, from satisfying personal curiosity to helping make informed medical decisions:

Personal curiosity and family interest: It is natural for families of twins, triplets or more to be curious about their genetic makeup and strive to better understand their unique bond. It will also support them in answering common questions such as “are they identical?” or “how do you tell them apart?”.

Knowing if the siblings are identical or non-identical can also inform family planning, as fraternal siblings are more likely to run in families, influenced by genetic factors. In contrast, identical siblings develop at random as the splitting of a fertilised egg occurs spontaneously.

Health purposes: Testing allows families to ensure that their medical records are accurate and up to date, providing clarity for the future.

It can also be incredibly useful in understanding compatibility for blood donations or organ transplants. Identical siblings are a near-perfect genetic match, which means there’s a much lower risk of rejection in the case that a transplant is needed, often reducing or eliminating the need for immunosuppressants.

Recognising individuality: Zygosity testing encourages families to explore what makes the siblings unique, regardless of whether they are identical or fraternal. Even though identical siblings share 100% of their genetic makeup they often have different personalities, interests and emotional responses, shaped by their environment and experiences.

Research studies: Zygosity testing can be instrumental in research studies by providing accurate information about their genetic makeup. This is used to support and guide an understanding about the influence of genetics and the environment on various traits and diseases. It can also be helpful in determining risks to perinatal health and child development.

How is zygosity testing performed?

Completing a NorthGeneTM zygosity test is incredibly simple and can be done in the comfort of your own home:

  1. Order your at-home test kit online.
  2. Complete the sample request form provided with all relevant information.
  3. All children involved in the multiple birth will need to provide a DNA sample. To take the DNA sample, a test swab will be rubbed a few times on the inside of the cheek. This collects a small sample of cells which is needed to extract the DNA and run the test. (Two sterilised test swabs for each person involved are included in the kit).
  4. Once all samples have been taken, place them back into the envelopes and seal in the tamper-proof bag supplied. The postage has already been pre-paid, ready for you to return to NorthGeneTM via your nearest post box or post office.
  5. Results will be sent to you via email within 1-2 working days.

How accurate is zygosity testing?

NorthGeneTM compares 24 genetic markers to conclude if siblings are identical or fraternal. The test is completed with a high accuracy rate of over 99.9%.

However, it must be noted that if multiple individuals were involved in the birth, but not all of them can provide a DNA sample, we will only be able to provide the relationship between those tested.

Additional support available from NorthGeneTM

We are proud to work closely with the Twins Trust, an organisation who provides families of multiple birth children with information that helps inform decisions and advocates community support.

To contact the Twins Trust for further information or guidance, please find their details below:

📞 01252 332344
📧 enquiries@twinstrust.org

Zygosity testing with NorthGeneTM

Zygosity testing helps families connect, plan for the future and celebrate the individuality of each child. Whether you’re curious or making important health decisions, knowing whether you or your children are identical or fraternal can be meaningful in more ways than one. NorthGeneTM is committed to providing accurate and reliable testing with compassion and care.

Contact our Customer Services team today to learn more and order your at-home zygosity testing kit.

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