Twin Zygosity Test.
This test is performed to definitively show whether sets of twins are fraternal or identical.
(Definition of zygote:  "A  zygote is the product of the fusion of an egg and a sperm. It contains two copies of each chromosome, one from each parent. Egg and sperms cells, on the other hand, each contain only one copy of each chromosome. The zygote develops into an embryo.")
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Usually when twins are born, the physician is able to tell whether twins are identical or fraternal by examining the placenta.  Identical twins usually share a placenta, while fraternal twins are usually in two different placentas.  Fraternal twins will not have exactly the same DNA, although like other siblings, they will share some of the DNA they inherit from both parents.
Records can be lost, however, and the placentas may have been discarded or damaged before twin zygosity was determined, or doubt may arise because of the twins' physical characteristics.  In these cases, only a DNA test will be able to reveal the truth.  A twin zygosity test compares the twins' DNA profiles to see whether they are a match.  An exact match proves they are identical.
A twin zygosity test may be used to satisfy personal curiosity as well as to help solve health problems for the future.  For example, in case of one twin needing an organ or tissue transplant donor, the identical twin is a perfect choice.
Fraternal twins are the product of two eggs and two sperm cells.  A set of fraternal twins can have two different fathers.  Sperm cells can live in a woman's body from 4 to 5 days.  Once released by an ovary, an egg cell remains viable for 12 to 48 hours.  Should a woman have sexual relations with more than one man during this fertile period and release two egg cells while she is ovulating, she could become pregnant with fraternal twins who have two different fathers.
There are even rare cases where assisted fertility methods such as in vitro fertilization (IVF), can produce fraternal twins with different fathers.  Should the sperm sample used in an IVF case inadvertently contain a mixture of different donor's sperm, any resulting zygotes could have different fathers.  While this is a rare occurrence, it has happened.
It is for this reason that we recommend both fraternal twins involved in a paternity case be tested against the alleged father. Unlike fraternal twins, identical twins cannot have two different fathers, as identical twins are the product of one egg and one sperm cell whose zygote splits early in its development.  Since only one zygote was formed in fertilization, only one man can be the father.
Requires samples from Twin A and Twin B.  Turnaround averages 5 business days from receipt of all samples.
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