“Are my twins Identical or Fraternal?”
This is the most often asked question by parents of multiples.
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”, which 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.
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 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.
Private DNA testing is not available to New York residents due to New York state law. Legal DNA testing may be performed if ordered by a physician or attorney.
How much does a Twin Zygosity test cost?
When ordering a twin zygosity test, you may choose to use a testing kit from Bio-Gene DNA or our Make Your Own DNA Kit option.
All Bio-Gene DNA Test Kits are shipping via USPS Priority Mail. If those being tested live in different locations we can ship a test kit to each person. A unique identification code is assigned to each case and this code would be the same for each kit shipped. The samples would be matched in our lab prior to testing.
Our Make Your Own DNA Kit option may be especially helpful if you are using alternate samples and do not need to have a kit sent to you, or if you are facing time constraints in needing the results and do not wish to wait for a kit to arrive. Upon ordering the Make Your Own DNA kit, you will be sent an email with your case number, detailed instructions, and the address to send your samples to.
Performing the Zygosity DNA test
We perform the testing by using 25 DNA markers to examine and compare each sibling’s DNA. Identical twins will match on all 25 markers. If 24 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 zygosity DNA test.
The most accurate method of testing is to use a buccal swab, 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 and in these circumstances we are able to test alternative or non-standard samples. The sample must have been stored in a paper envelope or breathable environment and not in a plastic bag to allow the greatest opportunity for the sample to be viable during testing.
To learn more about using non-standard items for DNA testing, please click HERE.