Zygosity

We receive many questions about zygosity—that is, whether you and your twin are identical or fraternal. We hope you find the following information helpful. If you still have questions that aren’t answered here, send an email to ws.twinregistry@wsu.edu.

Identical vs. Fraternal Twins

There are two types of twins: identical (also called monozygotic, abbreviated MZ) and fraternal (also called dizygotic, abbreviated DZ). A zygote is the fertilized egg formed when a sperm combines with (fertilizes) an egg at conception. The zygote then starts to divide and forms an embryo. The embryo is called a fetus in later stages of prenatal (before birth) development.

Monozygotic means “one zygote.” MZ twins develop when one egg is fertilized by one sperm to produce a single zygote. The zygote starts to develop into an embryo, but at some time during the first two weeks after conception, the developing embryo splits into two identical parts. Each part develops into a baby. The two babies are genetically identical: they share 100% of their genetic material (genes, made of DNA) and are always the same sex.

Dizygotic means “two zygotes.” DZ twins develop when two eggs are released at the same time, and are fertilized by two different sperm. These two zygotes are each implanted in the womb (uterus) and develop at the same time. On average, DZ twins share half their genetic material, which is the same amount shared between any pair of siblings. In fact, it has been jokingly said that the only difference between DZ twins and non-twin siblings is that DZ twins have been “womb-mates” for nine months. DZ twins may be of the same sex or opposite sexes.

How is Zygosity is Determined

Twins often want to know their zygosity (whether they are MZ or DZ twins). While opposite-sex twins are always DZ, the zygosity of same-sex twins may be difficult to determine. The only way to be sure is by doing DNA zygosity testing.

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. These regions are carefully selected, because all humans share mostly similar DNA.

Zygosity testing can be done on cells found in cheeks, blood, amniotic fluid, and other tissues. A twin zygosity test compares the twins’ DNA profiles at a number of markers to see whether they match. If many of these markers are the same in two twins, then it is highly probable (i.e., greater than a 99% chance) that they are identical twins. If the twins are different at one or more markers, they are considered to be fraternal twins. The more markers studied, the more reliable the zygosity results are.

The laboratory techniques used to determine zygosity are very similar to those used in forensics and in paternity tests. The DNA locations that are analyzed are not related to any specific physical trait such as hair color, height, or personality. Nor are they locations that might indicate whether a person is at risk for a genetic disease.

Zygosity Determination Done by the Twin Registry

The Washington State Twin Regsitry uses a form of zygosity determination called “self-report.” This determination uses the responses that you and your twin provided to two questions on the registry recruitment survey: “When you and your twin were children, were you as alike as two peas in a pod or of ordinary family resemblance?”; and “How often did parents, relatives, teachers, and strangers have trouble telling you and your twin apart?”

Based on the answers that you and your twin provide and comparing your answers with those of other twins in the Registry, we have developed a method that can predict zygosity with about 96% accuracy. We use this “self-report” zygosity to generate the statistics for our website and to recruit twins for our studies. This method is statistically useful as it accurately reflects group trends and tendencies.

The Importance of Zygosity Testing

For research purposes, scientists need to distinguish between MZ and DZ twins. If they are studying genetic influences on health, it is very important to know if they are working with twins who share the same genetic make-up.

For twins themselves, the biggest reason might be curiosity, or to know the answer to the common question, “Are you identical or fraternal?”

As with any family member, you might also be curious about how your twin’s health relates to your own health. In addition, you might eventually encounter some medical situations where knowing your zygosity could be important. For example:

If one twin needs an organ transplant, the best donor would be his or her MZ twin, since there is an extremely low chance of organ rejection.
If one twin needs a blood donation, his or her MZ twin would be the perfect blood donor since both twins have identical blood types.
If one twin is diagnosed with certain diseases, the chance that the other twin might eventually have the same disease is high if they are MZ twins. Depending upon the disease, doctors can suggest possible ways to prevent it in the unaffected twin, or they may be able to test for the disease to diagnose it early and avoid possible complications.

If you would like additional information about how zygosity findings might affect your personal health, contact your health care provider or a genetic counselor. Genetic counselors may be located through the National Society of Genetic Counselors or the American Board of Genetic Counseling.

Zygosity Testing

Are you curious about whether you and your twin are identical or fraternal? If you are a male/female twin pair, you are definitely fraternal twins. However, if you are twins of the same sex, you might not be certain.

If you and your twin need zygosity results that are suitable for health-related purposes, please follow up with your healthcare provider to have a test performed by a CLIA-certified* laboratory.

If you want to know your zygosity only for curiosity and not to make medical decisions, there are commercial laboratories that offer direct-to-consumer zygosity testing. “Direct-to-consumer” means that you deal directly with the laboratory, without necessarily involving a doctor or an insurance company. The laboratory will mail you a home collection kit that you can use to collect cells from inside your cheeks. The collection process is easy and non-invasive. After you collect the cells, you will mail them back to the laboratory in the container provided. The laboratory will extract DNA from your cheek cells and use it to create your DNA profile.

A twin zygosity test compares the DNA profiles of a pair of twins at a number of markers (well-studied sequences of DNA) to see whether they match. If many of these markers are the same, then it is highly probable (i.e., greater than a 99% chance) that they are identical twins. If the twins are different at one or more markers, they are considered to be fraternal twins. The more markers studied, the more reliable the zygosity test results are.

Below we provide a partial list of laboratories that offer direct-to-consumer zygosity testing. The Twin Registry does not endorse any of these laboratories; we are simply providing examples of places that offer this service.

DNA Diagnostics Center (DDC)
Turnaround time for results: 3 working days
CLIA certified?*: Yes
Cost: $250/pair

23andMe
Turnaround time for results: 3-4 weeks
CLIA certified?*: Yes
Cost: starting from $79/person

LabCorp
Turnaround time for results: 3-5 days
CLIA certified?*: Yes
Cost: starting from $210/pair

*CLIA certification: sets certain federal standards for quality and competence. In the US, clinical laboratories must be CLIA-certified. Only testing done in a clinical laboratory (CLIA certified) should be used to make a medical diagnosis, or to develop a plan for prevention or treatment. Commercial zygosity testing laboratories do not have to be CLIA-certified, although many are, because they also offer other testing that can be used for medical decisions. (CLIA = Clinical Laboratory Improvement Amendments)

Common Misconceptions about Zygosity

The question of whether a pair of twins is identical or fraternal is one that many parents and twins have. If the twins are opposite sex, or very different in appearance (e.g. one is blonde and blue-eyed, one is brunette and brown-eyed), this question has a fairly obvious answer. However, fraternal twins can look very similar, and identical twins can look different. For twins falling into these categories, this can create a lot of confusion and uncertainty when asked if they’re identical or fraternal.

We’ve heard the following statements from twins who were later determined to be identical:

The doctor told our mom that we are fraternal because there were two sacs or two placentas
We look nothing alike
We were different sizes at birth so we can’t be identical
We’re so different we can’t be identical

Twin Pregnancies

The number of amniotic sacs or placentas present at birth has been a major source of confusion, especially in older twins whose mothers would not have had an ultrasound performed during pregnancy. However, even in younger mothers, dichorionic-diamniotic (di-di) pregnancies still cause uncertainty in twin type. 30% of di-di pregnancies are identical twins, and many doctors still believe that di-di pregnancies can only result in fraternal twins. The following image shows the four different types of twin pregnancies. All four figures show identical twins, but the first two figures also apply to fraternal twins, explaining why there can be confusion in classifying twins at birth.

Monochorionic-monoamniotic twin pregnancies are very high risk, as they share a placenta and an amniotic sac, which can result in cord entanglement and/or twin-to-twin transfusion syndrome (TTTS). TTTS can result in size differences at birth as blood flows unevenly between the twins. Positioning of twins within the womb can also cause size differences in identical twins.

Epigenetics

Identical twins who are discordant (only one has it) for a health condition have helped make contributions to the growing field of epigenetics. Living creatures have millions of chemical tags which can attach to DNA. These tags regulate cell activity, modifying the DNA without changing the actual DNA itself, which in turn affects gene activity.

These tags can be turned on or off by things such as diet, physical activity, stress, sleep, and environmental exposures. Errors in this epigenetic process have been found to contribute to cancer, metabolic disorders, and degenerative disorders. Epigenetic differences can also lead to differences in appearance, particularly in female twins.

To about genomics.

At-Home DNA Testing

The increasing popularity of at-home DNA tests is also leading to confusion and uncertainty. These tests may show that you and your twin are 99.9% identical but that ancestry results differ, leading you to believe that you are not identical.

This is Incorrect!

A comparison study of DNA kits from different companies done by the CBC in Canada showed that genetically identical twins do not obtain the same ancestry breakdown.

According to Dr. Simon Gravel, a population geneticist, consumers should take the results generated by these tests with a grain of salt. People need to understand these tests are not subject to the same standard as diagnostic medical testing. They are more like a “recreational scientific activity,” he said.

So, if your 23andme results come back as identical, but your ancestry results are not the same, you’re still identical.

Still Have Questions?

If you still have questions about zygosity, please email ws.twinregistry@wsu.edu.