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These questions and answers provide basic information about the occurance of identical and fraternal twins. We also discuss the circumstances where twin zygosity should be tested. Listed below is a selection of those that are most frequently asked. If you can not find the answer to your question, please contact us and we'll promptly provide you with an answer.

What is zygosity?

What is a zygosity test?

What is the need for a twin zygosity test?

What causes a fertilized egg to split causing identical twins?

What is the rate of natural identical twins?

What is the rate of natural fraternal (dizygotic: DZ) twinning?

What are the sex ratios for identical and fraternal twins?

What is the effect of assisted reproductive technologies (ART) on rates of twinning?

My physician already determined that the twins are fraternal. Why have a DNA test?

My physician already determined that the twins are identical. Why have a DNA test?

Can different-sex twins be identical?

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What is zygosity?

Twin zygosity is the genetic relationship between two twins (or higher order multiples such as triplets and quadruplets, etc.). With respect to genetic similarities or dissimilarities, twins are classified as identical or fraternal. More correct terminology should refer to identical twins as monozygotic (or MZ) while fraternal twins should be referred to as dizygotic (or DZ).

When one egg is fertilized by a single sperm cell and then splits, two identical or monozygous twins will result. These twins are referred to as identical since the sequence of the twins' DNA is essentially identical. Identical twins must be of the same sex. Importantly, the term "identical" often does not apply to the actual physical or behavioral similarities between identical twins. However, identical twins tend to be very similar for traits such as hair color, hair texture, eye color, height, and weight. When two eggs are independently fertilized by two sperm cells, fraternal or dizygous twins result. Fraternal twins do not have identical DNA but will have half of their DNA in common, just like siblings of the same parents that are born at different times. Fraternal twins can either be of the same sex or different sex. back to top

What is a zygosity test?

Since the above definition of an identical or fraternal twin refers only to the similarity or dissimilarity of the twins' DNA, a zygosity test is a comparison of the DNA between two twins. At Proactive Genetics, we analyze regions of DNA which are called microsatellites. These are stretches of DNA that repeat over and over again, like AACT AACT AACT AACT. All humans have repeating stretches of DNA, and the repeats vary from one individual to another. If several of these repeat regions are the same between two twins, then it is highly probable (i.e., greater than a 99% chance) that they are identical twins in that the test fails to prove they are fraternal. If the twins are different at one or more repeat markers, they are considered to be fraternal twins. Interestingly, these are almost the exact same techniques used in forensics and in paternity tests. The genes analyzed are not related to any specific physical trait such as hair color, height or personality. back to top

What is the need for a twin zygosity test?

Most people desire a zygosity test to satisfy their curiosity. However, 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. For important medical decision making, we strongly suggest obtaining a test with an accredited lab which includes supervised sample collection and a chain of custody. We would be glad to provide a referral. back to top

What causes a fertilized egg to split causing identical twins?

The exact reasons that this occurs are not known. back to top

What is the rate of natural identical twins?

The rates of twinning for identical (monozygotic; MZ) twins are relatively constant worldwide at about 3.5 / 1000 births. back to top

What is the rate of natural fraternal (dizygotic: DZ) twinning?

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 (dizygotic) and 30% are identical (monozygotic). back to top

What are the sex ratios for identical and fraternal twins?

For fraternal twins, it is equally probable that both twins would be like-sexed (girl/girl or boy/boy) as unlike-sexed (girl/boy). Similarly, for identical twins it is equally probable that they would be either both male or both female. back to top

What is the effect of assisted reproductive technologies (ART) on rates of twinning?

Since multiple embryos are often transferred, there has been a huge explosion in twinning rates in many developed countries. Interestingly, 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. back to top

My physician already determined that the twins are fraternal. Why have a DNA test?

Interestingly, about 25-33% 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 or other blood test was performed, it is difficult to definitively determine if like-sexed dichorionic twins are fraternal. A DNA test is the easiest way to resolve this potential confusion. back to top

My physician already determined that the twins are identical. Why have a DNA test?

When a pathologist or physician correctly examines the membranes surrounding the twins and determines that the twins were monochorionic, the twins are always identical or monozygotic (MZ), regardless if the amniotic membrane is shared (i.e. monoamniotic) or non-shared (i.e. diamniotic) and a twin zygosity test is not warranted. Sometimes the placental examination is skipped or made difficult by the delivery, especially if there was a cesarian section. If chorionicity is unknown or uncertain, a DNA test is the best way to definitively know if like-sexed twins are identical or fraternal. back to top

Can different-sex twins be identical?

No. Different-sex twins are, by definition, fraternal twins. Only same-sex twins can be either fraternal or identical. back to top


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Our customers say...

"We just got the lab results that says our girls are identical. Even though I lost the office pool over the conclusion, I want to say how interesting and *fun* this was to do. Our doctor told us they were likely fraternal, but not even grandma could tell them apart. The issue was a matter of constant debate. I've already refer one..."

-Stasia (Redding, CA)

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