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Identical Twins or Fraternal Twins

identical twin testMost people assume that being a twin means having a similar physical appearance. However, twin type is determined long before twins are born. The terms identical and fraternal are common words that refer to “zygosity." 

What is "Zygosity"?

Zygosity is the characteristics of the cell union that happen at conception. Monozygotic, which means “one zygote,” refers to Identical Twins. Twins form when a single fertilized egg splits into two genetically identical parts. The twins share the same DNA set, and therefore they usually share many similar features and attributes. However, since physical appearance is  also influenced by environmental factors and not just genetics, identical twins can actually look very different. Identical twins are always same-sex sets.

Dizygotic means “two zygotes” and refers to twins who are Fraternal. Twins develop when two separate eggs are fertilized and implant in the uterus. The genetic connection is no more or less the same as siblings born at separate times. They sometimes look alike, or they may not look similar at all.

Identical vs Fraternal Triplets

Identical triplets are born as a result of  a monozygotic pregnancy where there are three fetuses from one egg. Triplets that include identical (monozygotic siblings in addition to a fraternal sibling are the result of a dizygotic pregnancy. This is where one zygote divides into two fetuses and the other doesn't.

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Twin Zygosity Test FAQ's

A twin zygosity DNA test is 100% accurate. All genetic markers must match exactly to be monozygostic (identical).   Our labs run all DNA tests twice to confirm results, and the answers we provide are 100% accurate based on the DNA profiles of those being tested.

It takes 1 business day to complete testing and send results for standard DNA twin testing, once the lab has received all tested parties’ samples. 

Your twin zygosity DNA test results will be sent to the email address that was provided on order form. They are sent securely and are in a PDF format.  You will need a PDF reader to view them. We suggest Adobe reader which is secure and free to download. Your results can be opened and viewed, printed, or forwarded depending on your choice to share the information. Only the ordering party, or designated email(s) will receive a copy of the report.

We also provide a booklet that explains in more detail some of what you may see on your report. 

USPS Priority mail is our primary method of sending printed sibling test results, if requested at the time of the order. 

We maintain your privacy every step of the way. All results and information are deleted from our system 30 days after the results are delivered to you (we keep the reports for 30 days in case you have any questions). We never sell or give your information to anyone, including government and private/public databases. No one will ever know this test was done unless you provide them the information.

DNA testing is not covered by health insurance or Medicaid because it is not a “medically necessary procedure.” 

Scientists can determine the zygosity of twins by using a DNA sample from each twin to compare their genetic markers. Markers are well-studied sequences of DNA. A laboratory can analyze specific genetic markers that represent individual regions of DNA that may differ in fraternal twins.

To determine whether you have identical or fraternal twins, DNA centers perform twin zygosity tests. Twin zygosity DNA tests are performed anytime after birth, in which a swab from the cheeks of each twin is taken. DNA gets taken from the samples, and then is analyzed and compared.

Because fraternal, or dizygotic, twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2.

Yes! Identical twins came from the same sperm and egg, so they have the same chromosomes and genes. But there are environmental differences that can affect the way they look and behave.  As identical twins get older they may look more and more different, because they are exposed to more diverse environments.

No! Studies have concluded that, even though the fingerprints of identical (monozygostic - MZ) twins may be very similar, they are not identical. MZ twins share the same genetic makeup (DNA) because they are formed from a single zygote (fertilized egg). However, fingerprints are not an entirely genetic characteristic.

Yes, however identical triplets or quadruplets are very rare and result when the original fertilized egg splits and then one of the resultant cells splits again (for triplets) or, even more rarely, a further split occurs (for quadruplets). 


How can I tell if my twins are Identical or Fraternal?

This is the most common question among parents of multiples. As the birth rate of twins has risen 79 percent over the last three decades and continues to increase, this will continue to be a question that requires answers.

Since appearance isn't a reliable guide in determining twin type, how can parents confirm their twins' zygosity? In some cases, it will be evident during pregnancy. Some identical twins form in a single sac, sharing a single placenta and amniotic membranes. However, the number of placentas isn't always a clear indicator either. Two placentas of fraternal twins can fuse together and appear to be one. And identical twins may develop with completely separate placentas and sacs. The only way to know for sure is through genetic testing.

A Journey Genetic Testing Zygosity DNA Test is a simple procedure performed sometime after the babies are born.  Some parents choose to start testing right away, while others prefer to wait to initiate testing until the twins’ features emerge later in life. The test consists of a painless buccal cheek swab collected from each twin and then sent to our lab where the DNA from the samples is extracted, analyzed, and compared.

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Cost Number of People Testing Processing Time
$109 Twins 1 Business Day
$184 Triplets 1 Business Day
$259 Quadruplets 1 Business Day
+$125 Non-standard DNA Sample (per sample) +5 Business Days

Performing the Twin Zygosity DNA Test

We perform the testing by using 24 specific DNA markers to examine and compare each sibling’s DNA. Identical twins will match on all 24 markers. If 23 or fewer markers match, then they would be excluded as being identical twins. There are times the mother may be unsure whether the two children are from the same father. This can happen, even if the siblings were born together.  Should  this occur, we can perform a sibling DNA test (if requested) in conjunction with the Zygosity DNA test. This would only be done if the test proves that they are not identical twins.

Sometimes it is not possible to obtain a buccal sample from a person. In these circumstances we are able to test an alternative, or non-standard, DNA sample. To learn more about using non-standard items for DNA testing, please click HERE.


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