Identical Twins or Fraternal 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.
What is “Zygosity?”
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.
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 twin 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 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.
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 alternative, or non-standard, DNA sample. To learn more about using non-standard items for DNA testing, please click HERE.
Often Asked Questions About Twin Testing
Twin testing is also known as a twin zygosity test, and is performed to determine whether twins are identical or not identical through examination of the twins’ DNA. If the DNA profiles are different, then the twins are not identical (fraternal). When the DNA profiles are identical, Bio-Gene DNA reports a probability that the twins are identical.
Identical twins occur when a single fertilized egg splits after conception into two identical halves, each of which develops separately. Identical twins will have exactly the same DNA, which can be established through DNA testing. Identical twins are of the same sex and have the same eye and hair color, as well as blood type.
Fraternal twins, also called non-identical twins, are formed from two eggs, fertilized separately. Fraternal twins can be the same sex or opposite sexes. Their physical characteristics usually differ and their DNA always differs.
Approximately 30 out of every 1,000 births in the United States produce twins. One of every three sets of twins is identical.
In rare instances, fraternal twins can actually be half-siblings, with different fathers, even though they are born at the same time. Although the standard Bio-Gene DNA twin test does not include the establishment of paternity, it is possible to determine the paternity of twins by testing their alleged father(s) or possibly by conducting a sibling (full or half) analysis on the DNA data generated from non-identical twins.
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 answer to this question may be somewhat surprising. In DNA testing, a cheek swab is preferable to a blood test. The reason is that blood chimerism has been shown to exist between twins and the mother. What this means is blood stem cells can co-mingle between twins and the maternal circulation during development. This can cause some populations of blood cells to have an origin that is different from the person in which descendants of those cells are now circulating. For this reason, it is preferable to look at body tissues such as from the inside of the cheek which are not typically affected by this sort of chimerism.
Almost 33% percent of the time when there are two sets of membranes (i.e. dichorionic) identified at delivery, the twins are actually identical and not fraternal. If the fertilized egg splits prior to 2 days after conception, two complete placentas and two sets of membranes are formed causing dichorionic placentation. This is a common misunderstanding that leads to incorrect zygosity classification. This is true regardless if the dichorionic placenta is fused or not. Unless a DNA test is performed, it is difficult to definitively determine if like-sexed dichorionic twins are fraternal.
In studies regaring multiple ovulation (i.e. dizygotic twins), there is some evidence that twinning is heritable, but no reliable family studies, or molecular-genetic data, suggest that it is either autosomal or sex-linked. There is little evidence that suggest that identical twinning has any genetic basis.