FAQ - Twin and Triplet Zygosity

We receive many questions from parents of multiples and multiples themselves. 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 twin zygosity?

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?

What is a zygosity test?

How old must the multiples be to have the test?

How do we get DNA samples to conduct the test?

Can different-sex twins be identical?

What are mirror image twins?

What are polar body or "half-identical" twins?

Is having twins affected by heredity or is the twins gene, sex-linked or autosomal?

What is twin zygosity?

Zygosity is the genetic relationship between two twins. With respect to genetic similarities or dissimilarities, twins are classified as identical or fraternal.

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 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

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

Unless a DNA or other blood test was performed, it is difficult to definitively determine if twins are fraternal. Interestingly, about 25-33% percent of the time when there are two sets of membranes identified at delivery, the twins are actually identical and not fraternal. This is a common cause of incorrect zygosity classification. 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 trained physician correctly examines the membranes surrounding the twins and determines that the twins were monochorionic, the twins are always identical. Sometimes this examination is skipped or made difficult by the delivery, especially if there was a cesarian section. In this case, a DNA test is the best way to definitively know if the twins are identical or fraternal. 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. 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 cases where DNA from a suspect is compared to DNA at the crime scene. If the samples are collected in the right way, then this test is very accurate. Indeed, many people have been convicted on DNA evidence alone. back to top

How old must the multiples be to have the test?

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. back to top

How do we get DNA samples to conduct the test?

It's very easy since the same DNA is found in every cell in our body. We will send regular, cotton-tipped swabs and a vial containing buffer solution through the mail. The twins should rub the inside surfaces of their mouths with the cotton swab, painlessly removing cells from the surface of the cheek and gum. The swabs should then be placed in the buffer solution included with your kit and mailed back in the envelope we provide. We will remove the DNA from these cells and purify it to analyze the DNA markers. 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

What are mirror image twins?

Mirror image twins are identical twins that result when a fertilized egg splits later in the embryonic stage than usual, typically from day 9 to 12. Mirror image twins tend to exhibit characteristics with reversed asymmetry (e.g., one twin is left-handed and the other is right-handed). If this split happens much later than this, then the twins can be co-joined. A DNA-based zygosity test cannot determine is twins are mirror image twins. back to top

What are polar body or "half-identical" twins?

Polar body twins are believed to occur when the mother's egg splits before fertilization and one half (referred to as the 'polar body') is also fertilized by a different sperm. This results in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). They share some features of identical twins and some features of fraternal twins and are so-called half-identical twins. However, DNA-based zygosity tests are currently not available to determine if twins are polar-body twins. back to top

Is having twins affected by heredity or is the twins gene, sex-linked or autosomal?

Although there is some evidence that twinning is heritable, neither family studies nor molecular-genetic data suggest that it is either autosomal or sex-linked. back to top