Twin Zygosity DNA FAQs

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Identical twins occur when a single fertilized egg splits after conception into two identical halves, each of which develops separately. Identical twins will have exactly the same DNA, which can be established through DNA testing. Identical twins are of the same sex and have the same eye and hair color, as well as blood type.

Fraternal twins, also called di-zygostic twins, are formed from two eggs, fertilized separately. Fraternal twins can be the same sex or opposite sexes. Their physical characteristics usually differ and their DNA always differs.

Approximately 30 out of every 1,000 births in the United States produce twins. One of every three sets of twins is identical.

The answer may surprise you. Yes, in very rare circumstances identical twins can have different blood types. This has been proven by testing we have done on a set of identical twins.

Yes. In rare instances, fraternal twins can actually be half-siblings, with different fathers, even though they are born at the same time. Although the standard Journey Genetic Testing DNA twin test does not include the establishment of paternity, it is possible to determine the paternity of twins by testing their alleged father(s) or possibly by conducting a sibling (full or half) analysis on the DNA data generated from non-identical twins.

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.

he answer to this question may be somewhat surprising. In DNA testing, a cheek swab is preferable to a blood test. The reason is that blood chimerism has been shown to exist between twins and the mother. What this means is blood stem cells can co-mingle between twins and the maternal circulation during development. This can cause some populations of blood cells to have an origin that is different from the person in which descendants of those cells are now circulating. For this reason, it is preferable to look at body tissues such as from the inside of the cheek which are not typically affected by this sort of chimerism.

Almost 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 test is performed, it is difficult to definitively determine if like-sexed dichorionic twins are fraternal.

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