Zygosity Testing For Twins

How Can I Tell If My Twins Are “Identical” Or “Fraternal”?

Twin Zygosity Test 100% Accurate

This Is The Question We Hear The Most From Parents Of Twins!

Most people assume that being a twin means having a similar physical appearance. However twin type is determined long before the twins are born. The terms identical and fraternal are common words that refer to “zygosity.” Zygosity is the characteristics of the cell union that happen at conception, which results is the genetic relationship between two twins. With respect to genetic similarities or dissimilarities, twins are classified as identical or fraternal.

When one egg is fertilized by a single sperm cell and then splits, two identical, or monozygotic, twins will result. These twins are referred to as identical since the sequence of the twins’ DNA is identical. Identical twins must be of the same sex. Importantly, the term “identical” often does not apply to the actual physical or behavioral similarities between identical twins. However, identical twins tend to be very similar for traits such as hair color, hair texture, eye color, height, and weight.

When two eggs are independently fertilized by two sperm cells, fraternal or dizygotic twins result. Fraternal twins do not have identical DNA but will have half of their DNA in common, just like siblings of the same parents that are born at different times. Fraternal twins can either be of the same sex or different sex. It is interesting to note that, although there is some evidence that twinning is heritable, neither family studies nor molecular-genetic data suggest that it is either autosomal or sex-linked. 

The only 100% accurate way to determine identical or fraternal twins is by a DNA Zygosity Test.

What Is A Zygosity Test?

Since the above definition of an identical or fraternal twin refers only to the similarity or dissimilarity of the twins’ DNA, a zygosity test is a comparison of the DNA between two twins. At Journey Genetic Testing, our lab analyzes regions of DNA which are called microsatellites. These are stretches of DNA that repeat over and over again, like AACT AACT AACT AACT.

All humans have repeating stretches of DNA, and the repeats vary from one individual to another. When several of these repeat areas are the same between two siblings, it is probable they are identical twins. If the twins are different at one or more repeat markers, they are considered to be fraternal twins. Interestingly, these are almost the exact same techniques used in forensics and in paternity tests. The genes analyzed are not related to any specific physical trait such as hair color, height or personality. 

Zygosity testing works on multiples of any age, newborns to adults. The simple and non-invasive swabbing procedure allows for the rapid and painless collection of a DNA sample.

The Need For Twin Zygosity Testing

Most people order a zygosity test to satisfy their curiosity, and that of their friends and family. However, there are some health conditions with where it would be useful to know if a condition experienced by one twin will be likely to be experienced by the other twin. Identical twins can donate organs to each other in certain situations. For important medical decision making, we recommend have a legal, chain of custody twin test done. Since the laboratory is AABB accredited, it just requires having an authorized DNA collector take the DNA samples, and complete a chain of custody. If you have a need for a legal document proving zygosity, please talk with one of our Genetic Consultants and we can set everything up for you.

Limitations of Zygosity Testing

There are two instances where Zygosity DNA testing has not developed to the level of accuracy we require. Because of this we are unable to perform the testing in these instances:

  • Suspected mirror image twins. Mirror image twins are identical twins that result when a fertilized egg splits later in the embryonic stage than usual, typically from day 9 to 12. Mirror image twins tend to exhibit characteristics with reversed asymmetry (e.g., one twin is left-handed and the other is right-handed). If this split happens much later than this, then the twins can be co-joined. A DNA-based zygosity test cannot determine is twins are mirror image twins. 
  • Polar body or “half-identical” twins. Polar body twins are believed to occur when the mother’s egg splits before fertilization and one half (referred to as the ‘polar body’) is also fertilized by a different sperm. This results in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). They share some features of identical twins and some features of fraternal twins and are so-called half-identical twins. However, DNA-based zygosity tests are currently not available to determine if twins are polar-body twins

Benefits Of DNA Zygosity Testing Through Journey Genetic Testing

Utilizing the most advanced technology. along with standardized methods in human identity testing, our results are guaranteed to be 100% accurate. Ensuring this level of accuracy and quality control is of the utmost importance to us.

Specialization in zygosity testing allows us to offer these services at a fraction of the cost of what many testing laboratories charge. We are able to provide you access to the highest quality genetic testing services that were once out of reach because of high cost.

Our clients receive their customized DNA collection kits in one to three business days, depending on location. Once the samples are returned using the prepaid overnight FedEx mailer (USA addresses only for the return mailer), the laboratory immediately begins testing, and provides the results in just 1 business day in 99.8% of our cases. These are emailed to you the same day they are released in an easy to ready, and understand, PDF report. You can even choose to have a duplicate copy of your test results sent by USPS Priority mail.

Our Client Twin Zygosity Test Reviews

Great service

Rated 5 out of 5
June 29, 2022

“Thanks for your response to all of our voice and e-mails. I’m sure it can be frustrating at times dealing with anxious customers pestering you for results. Your customer service has been impeccable and we will certainly recommend Journey Genetic Testing in the future.”



Rated 5 out of 5
June 24, 2022

“We just got our results and found out that we are identical. I think everyone thought we were except my mother’s doctor. We have always looked just alike. I want to say thank you. It’s like we received this wonderful gift, 50 years later than we may have liked, but still very much a gift. We just couldn’t imagine any twins being closer than we are, at least emotionally. We’re best friends. Once again, thank you so much.”


Happy With Your Service

Rated 5 out of 5
April 30, 2022

“We have received the results really fast. We are glad to be reassured that our girls are identical. Thank you.”


Excellent Experience

Rated 5 out of 5
February 16, 2022

I got test results back from an ancestry company but they could not give me solid answers. I spoke with one of your representatives who explained the difference and why I should use a person to person DNA test. Your process was simple and easy to understand, and the results came back in just two days, rather than weeks like the other test I took. Thank you for explaining the difference and now I know for sure. It showed we were not related after all. Very relieved to know for sure. Thank you!


Thank you!

Rated 5 out of 5
January 5, 2022

Great job! Everything was fast, and just as you explained it to me. I am glad that I went with your company.

error: Our content is protected by copyright.