What does the DNA Zygosity Twin test tell us?

The zygosity test basically tells us whether the twins in question are monozygotic (identical twins) or dizygotic (fraternal twins).  Zygosity is a generic term which encompasses both types of twins and essentially simply refers to the degree of similarity between the twin genomes.  Whilst DNA profiles in dizygotic twins (Fraternal twins) can approximate each other, the match between the two twin profiles can however, never be identical.  Thus, it is only monozygotic twins (identical twins) that will have identical DNA profiles.

Identical or fraternal – how are they made?

The scientific terms themselves-monozygotic and dizygotic- explain the nature of fertilisation in the two types of twins as it happens at conception.  Monozygotic twins are born when one sperm cell (hence, the prefix mono), fertilises one egg and the egg then splits to form two independent embryos. On the other hand, dizygotic twins are formed when two sperm cells (Hence the prefix di) fertilize two separate eggs and both are simultaneously implanted in the uterine wall.

Why perform a Twin DNA Zygosity Test?

The Twin Zygosity DNA test is often done simply out of curiosity as either parents of twins, or twins themselves may have their interest piqued and wish to find out if they are identical or not .  However,  more importantly, in the case of the need for organ transplants, identical twins offer the highest success rate, reducing dramatically the chance of the organ being rejected.  This is due to the identical genetic make up and tissues type of identical twins.  The same applies to blood transfusion- identical twins have the same blood group, and thus, should the need arise, one becomes the life line of the other.

Twin Zygosity DNA test and Paternity DNA test

When a paternity test is requested to verify paternal lineage, it is customary to ask for both twins to be sent for testing.  This is due to the possibility of the mother having slept with two men within a short time frame and thus, the chance of two separate ova (female eggs)  being fertilised by two sperm cells, each sperm from a different man.  Under such circumstances, the children will have different fathers; hence, it is important to determine whether the twins are identical or fraternal.

Moreover, paternity DNA tests might prove inconclusive if the alleged fathers are identical twins as their DNA profiles will be identical replicas of each others’.  The child’s father in this case cannot be found through a DNA paternity test.

The Missouri identical twin fathers case

Identical twin brothers became father and uncle to a girl- but exactly which twin has what relationship to the girl?  Each twin had been simultaneously having an affair with the mother of the girl and both slept with her on the same day.  Identical twin father means that DNA paternity tests were of little use to solve the father or uncle dilemma- both would have equal probability of being the father.  The mother’s testimony was the only evidence that the court could rely on.  The story clearly shows up the limitation of current DNA testing.

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