Twin Zygosity Testing

 Twin Zygosity DNA Testing

 

Are my twins identical or fraternal?

This may be the most often asked question by parents of multiples. Most people assume that being a twins 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”, which is the characteristics of the cell union that happen at conception.

Monozygotic, which means “one zygote”, are Identical. 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 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.

Because 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 Twin 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 compared for similarities. There is a 99.99% accuracy rate for this testing, which is as high a result as scientifically possible.

Reasons To Consider a Twin Zygosity DNA Test

Some parents are motivated by medical concerns, while others are participating in scientific research. But most simply want to satisfy their curiosity or confirm their suspicions.

  • In the event medical concerns may arise. 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.
  • Records from birth may be lost or inaccurate.
  • The twins’ placentas may have been discarded or damaged before twin zygosity was determined, especially in the event of an emergency or health-threatening birth experience.
  • Different physical traits emerge as the twins grow, casting doubt on an original zygosity determination earlier in life.
  • Personal curiosity.

 

Interesting facts about twins

 

  • The rates of twinning for identical twins are relatively constant worldwide at about 3.5 / 1000 births.
  • Nigeria has the highest rate of natural fraternal twins at about 44.5 / 1000 births. Japan has the lowest rate of natural fraternal twinning at 2.3 / 1000 births.
  • The differences across populations for fraternal twin rates are due to different genetic predispositions for the simultaneous ovulation of more than one egg caused by different amounts or responses to follicle stimulating hormone (FSH).
  • For European-North American population of spontaneously conceived twins, roughly 70% are fraternal and 30% are identical.
  • The sex ratios for fraternal twins is equally probable that both twins would be like-sexed (girl/girl or boy/boy) as unlike-sexed (girl/boy).
  • The effect of assisted reproductive technologies (ART) on rates of twinning has caused a a huge explosion in twinning rates in many developed countries since multiple embryos are often transferred.
  • About 10-12% of ART associated pregnancies result in identical twins. It is believed that in-vitro fertilization (IVF) causes a physical disruption which results in the separation of two early cell masses.
  • Twin classification done at delivery often results in misidentification. For example, approximately 25-33% of identical twins have two sets of chorionic membranes. Unless a DNA is performed, it is difficult to definitively determine if like-sexed dichorionic twins are fraternal.

 

Performing the DNA test

We perform the testing by using 26 DNA markers to examine and compare each sibling’s DNA. Identical twins will match on all 26 markers. If 26 or fewer markers match, then they would be excluded as being identical twins. In the event that the mother is unsure whether the two children are from the same father (which can happen, even if they were born together), we can perform a Sibling DNA Test in conjunction with the Twin Zygosity DNA Test at a nominal extra charge.

Read why Bio-Gene will not perform 16 marker testing for any Twin Zygosity DNA  testing.

The most accurate method of testing is to use a buccal swab, which is similar to a “Q-tip”, which has been rubbed on the cheeks of the mouth of those being tested. Sometimes it is not possible to obtain a buccal sample. We are able to test alternative samples, normally at no extra cost. The sample must have been stored in a paper envelope or breathable environment. The sample should not have been stored in a plastic bag at any time.

Some of the alternative samples that can usually be tested include:

Amniotic Fluid

Cups / Bottles

Feminine Hygiene

Razors

Bandaid (Used)

Dental Floss

Fingernails

Tissue

Chewing Gum

Earwax

Hair (With Root)

Tooth-brush

Cigarette Butt

Electric Razor

Hat (Well Worn)

Toothpick

Condom (Used)

Envelope / Stamp

Pacifier

Underwear

 

What the test results will look like

When performing a Twin Zygosity test there are only two conclusions – either identical or fraternal. Below is a sample report performed showing that all genetic markers matched and therefore the twins were identical. If even one of the tested areas show a different number, then the result is automatically classified fraternal twins.

Twin Testing Result

As you can see, the results are conclusive and easy to understand. These results are always kept confidential and available to you 24 hours a day. If there are any questions, one of our professional and experienced Case Managers will be happy to assist you during regular business hours.

How to Order an Twin Zygosity DNA Test

To order your Twin Zygosity DNA test kit, please complete the order form below and you will be taken to our secure server. Our prices can be found by selecting the test of your choice, the number of people testing, and your payment preferences when providing the information that is relevant to your particular case.

When ordering an Twin Zygosity test you may choose to use a testing kit from Bio-Gene DNA or build your own kit. This may be especially helpful is you are using alternate samples, or do not wish to the kit to arrive due to time constraints. To learn about using your own DNA testing kit, please click HERE.

If you chose to use a Bio-Gene kit, you will choose the shipping option that is right for you. Once your kit is mailed, you will be provided a tracking number to be able to determine when it will arrive. We do not ship kits without a tracking number to protect your personal information.

If you choose to use your own kit, you will choose the “own kit” option when choosing the shipping options, and upon receipt of your order you will be e-mailed a submittal form, a unique ID number that relates to your case (we cannot process any test without this), instructions on how to collect the samples, and our lab’s address to send your completed kit with samples for immediate processing.

If you have questions please e-mail us at twins [at] biogenednatesting.com, or call one of our Case Managers at 1-888-361-5256 to discuss your case confidentially.