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What does Zygosity mean?

What does Zygosity mean?


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There are factors when twins are born that can help the delivering doctor determine if they are fraternal or identical. One of those factors is the placenta’s appearance, but that doesn’t provide a definitive answer. A twin zygosity test can take the comparison a step further and provide that definitive answer. 

Zygosity refers to identical twins coming from the one egg, a zygote, when early in the developing phase, the egg splits, forming two embryos. The twins born from this zygosity, coming from one egg that mated with one sperm, will have the exact same DNA. To learn more about a twin zygosity test in [nw_data field=seo_city], [nw_data field=state], connect with us by dialing (800) 219-4362.

What is a Zygosity test?

Twin zygosity testing can be done during prenatal or after birth. It evaluates DNA marker patterns concerning siblings to evaluate whether persons from multiple gestations, such as twins, triplets, are identical (monozygotic) or fraternal (dizygotic).

technician testing dna

What causes an egg to split into twins?

At the very beginning of development, one egg is fertilized by one sperm and forms identical twins, then the zygote splits in half. At this time, the zygote is a cluster of only a few cells. The division happening this early, each baby has precisely the same genetic information. 

When an expectant mother undergoes fertility assistance, identical twins can occur when one of the fertilized eggs is returned to her uterus, if that egg divides in half. This will create identical twins. 

Country, ethnicity, or race has no bearing on identical twins. Every couple has the same chance of having identical twins, regardless.  The odds of a pregnancy resulting in identical twins is one in every three sets. The remaining two-thirds are non-identical. 

How can you tell if twins are identical or fraternal?

  • Chromosomes: Identical twins are more than two individuals that look like each other. All babies possess XX or XY chromosomes, which distinguishes them as boys (XY) or girls (XX). A girl-boy twin set occurs when an X sperm fertilizes an X egg and a Y sperm fertilizes the other X egg. In rare cases are monozygotic twins are typically the same gender, therefore, it is safe to assume that a set of boy/girl twins are fraternal, which are the most common twin zygosity is approximately 40% of all twins born.
  • Blood Type: The blood type of multiples can assist in determining zygosity. The blood type of identical twins, (monozygotic) is typically the same, with some rare exceptions. The blood type of fraternal (dizygotic) twins could be different or the same. With this in mind, it is usually the conclusion that a set of twins with distinct blood types are fraternal. However, a set of twins with the matching blood type can be fraternal or identical. 
  • Development: A set of identical twins are developed from one fertilized egg, sharing the same placenta and typically two amniotic sacs. Fraternal twins are developed from two separate eggs and have individual amniotic sacs. 
  • DNA: A set of identical twins share the same DNA and typically develop at the same rate. Fraternal twins have separate DNA and develop differently. 

Do twins have same fingerprints?

Through twin zygosity test, the DNA fingerprints of dizygotic twins are used to identify them. If they do not match at least of two or more markers, the DNA fingerprints are a complete match, the twins are likely monozygotic. However, the complete genome of children cannot be determined solely by DNA fingerprinting. So, even with twin zygosity assessment, there is a slight possibility that twins, despite all DNA markers to match, are dizygotic.

How do you find Zygosity?

A twin zygosity test is with DNA. This is the most accurate process to determine zygosity. They take a swab from the mouth of each twin to be tested. The laboratory will use those swab samples to analyze the specific genetic markers of individual regions. It is important to note that the DNA of fraternal twins may differ.  

These regions are selected with extreme care because every human shares similar DNA.  Twin zygosity testing can be done on the cells found in amniotic fluid, blood, cheeks, and other tissues. Twin zygosity test does a comparison of the DNA profiles of the twins at several markers, looking for a match.

If there are many markers the same in a set of twins, the probability is 99% they are identical twins. If there is one marker different, we consider the twins as fraternal. The more markers the lab studies, more reliable the twin zygosity test results are.

sampling and testing dna


If twin zygosity test interests you, it is important to find a laboratory that is reliable and trustworthy. You can do a Google search for “twin zygosity test near meto gain a list, then research several before deciding. You can also ask your own medical professionals to recommend a testing laboratory or inquire if they do the testing in-house. Dial (800) 219-4362 to talk with us about twin zygosity test in [nw_data field=seo_city], [nw_data field=state].

June 2024