The Twin Zygosity DNA test can determine whether twins are identical or fraternal. Understanding whether a twin is identical or fraternal is not always easy due to possible similar physical resemblance even between fraternal twins. The only way to precisely confirm whether twins are identical or not is through the analysis of the DNA profile of the twins through DNA testing.

Identical or Fraternal?

Identical twins will have identical DNA profiles. Fraternal twins will have different DNA profiles and although they can be relatively similar they will not be identical. Therefore, once the DNA test is performed the profiles are compared to see whether they are identical or not.

Identical twins are formed when one egg is fertilised by one sperm, and then divides and separates into two identical cells creating two individuals with the same DNA profile.

Fraternal twins are born when two different eggs are fertilised by two different sperm. Hence the DNA profile will be different and the twins will be basically normal siblings born relatively close to each other.

When is the twin zygosity DNA test performed?

The twin zygosity DNA test is normally performed primarily for curiosity purposes as the parents or twins will want to confirm if they are identical or not. However, the results of the test may also be required for medical purposes to assess whether one twin can donate an organ or bone marrow to the other twin in the case of a transplant. In such cases, it is important to confirm with certainty whether they are identical or not to confirm the viability of the transplant.

Twin Zygosity testing and DNA Paternity Testing

When performing a DNA paternity test and twins are involved, we normally request that both children are sent for testing as we are not in a position to assess if the twins are identical or not. Technically, the possibility may exist that the mother may have slept with separate men on the same date or in proximity, and have had two separate eggs fertilised by sperm from different donors. In such cases, the two children will have different fathers.

Problems may also arise when determining paternity of a child through a DNA paternity test when the possible fathers are twins and hence have the same DNA Profile. In this case, the result for both twins will be the same and the paternity cannot be determined. Therefore in such instances the DNA paternity test will not be of any help.

A recent case which was highly publicised in the media involved a pair of twins from Missouri that were identified as the possible fathers of a little girl. Both fathers denied the paternity and requested a DNA paternity test – which of course could not distinguish the real father as they both had identical DNA. The court therefore had to decide based on the mother’s testimony.