The DNA lesson
"DNA, or deoxyribonucleic acid, is the building block of human genes," says Lesa Nelson, the lab supervisor at Affiliated Genetics. "Genes and variations in our DNA are what make each of us unique, and every cell, except red blood cells, contains a complete complement of our DNA." Picture it as a string of beads with each bead representing one of four different chemicals.

Like letters in the alphabet, each bead spells out unique genetic instructions. Each person's code contains about 3 billion different beads that spell out millions of different combinations. Just like a finger print, no two people's DNA is the same-unless they're identical twins.

When specialists evaluate the DNA samples, they compare the "bead strings" of each child. If the arrangements of the beads don't match, they know the multiples are fraternal. "Only identical twins have exactly the same DNA type, and they are the only individuals who can't be distinguished from one another," Nelson says. This type of twins is called monozygotic; fraternal twins are dizygotic.

Basis of Twins Testing
Testing the zygosity of twins, ie. whether they are mono- or dizygotic, involves analysis of bodily samples from both of the twins and often their mother and father. Individuals can be distinguished from one another by DNA fingerprinting. A DNA profile (or "fingerprint") consisting of eight genetic markers is constructed for each person. An analysis is done which compares the eight DNA markers, which are located in different regions of the human genome. At each of the DNA locations tested, a person will possess two genetic types (alleles). Each parent will pass one allele onto a child via the egg (mother) or sperm (father). The two types inherited from the parents thus make up the child's own unique DNA mix. Each person has their own pattern of markers - a DNA 'fingerprint.'

The DNA profiles (or "fingerprints") of the twins are compared. If their profiles are different, they are dizygotic twins with the same genetic relationship as siblings born at different times. If the twins' profiles are the same in all eight tests they are likely to be monozygotic twins.

(Please note: Since DNA fingerprinting does not attempt to match the entire genome (all the DNA) of the children, there is a small chance that a pair of twins is dizygotic despite matching at all the DNA markers used in the fingerprinting analysis.)

How is the testing done?
Most DNA testing companies send parents cheek swabs to gather cell samples. Unlike cotton swabs, these are topped with tiny round brushes. To collect cells, parents swirl the swabs inside each twin's cheeks for a minute, then prepare the samples to be sent to the testing company. The DNA is extracted from the swabs and tested. Twins are designated as dizygotic (fraternal) if their DNA fingerprint does not match at two or more of the DNA markers used in the analysis. If the DNA fingerprint of the children match completely, they are likely to be monozygotic(identical).

Cheek swabs are used by most DNA testing companies because they're convenient and easy for parents to use-plus they can administer the tests at home. Swabs are also soft and don't cause children anxiety or discomfort. When administered correctly, this form of DNA testing is more than 99% accurate.

Reference Interval:
Monozygotic "identical" twins: Greater than 99% certainty (all markers match).
Dizygotic "fraternal" twins: 99% certainty (two or more markers do not match).

When you get your results:
I want to touch on this, because there is often confusion about how the results are interpreted.

Testing 8 markers: I have many times seen the comment, "We had their DNA tested, and they shared 75% of their DNA, which means they must be half-identical." This is a false assumption. The twins are NOT half-identical (and even if they are, there is absolutely no genetic test that can determine this). What this result means is that 75% of the DNA markers *tested* are a match. In a routine zygosity test, eight markers are tested, so the twins most likely match for six. These "genetics" companies don't compare all the DNA; it would be WAY too time-consuming, not to mention very expensive. This has been explained above, but let's go over it again: The DNA testing company takes a sample of your children's DNA and looks at specific sequences at various locations on the DNA. In these locations are DNA "markers"—short DNA sequences that are useful for human identification. For each location, there are two copies—one inherited from the mother and one inherited from the father. These two alleles are sometimes identical (homozygous), but usually they are not the same size (heterozygous). The test reports the sizes of the DNA markers at each location. Every person inherits a unique combination of genetic markers from the biological parents.

Eight markers is a very common number to test (if there is a question – say, in a forensics lab - they can test more; there are companies who test up to 16). Markers are chosen by how often they "appear" in the general population. Some markers are more common, and some are quite rare. Of course, siblings are going to share similar DNA; after all, they got their DNA from the same two parents. So it’s expected some markers will match (same size) within families, but only identical twins will match for all eight markers. Please note that "matching" can include the absence of a marker in both children tested, but a certain percentage of markers do have to be present for the test to be accurate (and you may notice, some of these companies have disclaimers, as there is a less-than-1% possibility that fraternal twins will still match for all 8 markers and therefore be declared identical.)

Why is it important to know?
Many people just really want to know.

But testing for zygosity isn't just for fun. For medical reasons, it's important to know if your twins are identical. Because mono zygotic twins have the same genetic code, they may be able to help each other in extreme medical circumstances. Typically, identical twins have the same blood types, says Dr. Ann-Marie Gunteski, assistant laboratory director at DNA Diagnostics Center in Fairfax, Ohio. This means they could share blood in emergencies. In the case of organ transplants, it's always best to have a sibling donation. But with identical twins there is an even greater chance of organ acceptance. However, Dr. Gunteski warns there is no guarantee. "Identical twins are only genetically the same," she says. "Environmental factors, such as sun and pollution, can affect their individual cells differently."

Parents can also test their multiples' DNA for other safety reasons. In the unthinkable event of a disappearance or an abduction, DNA provides a more accurate form of identification than a baby's footprint. As frightening as it seems to prepare for the worst, confirming your twins' zygosity is important. Try to think of it like this: By testing them, you will be doing all you can to protect your children.

If anyone is still curious and wants to read more, here is an interesting and informative link on zygosity:

Twins Testing (Zygosity Testing)

Author: Cathy (2gherkins)