For years the dogma was that identical twins possessed DNA profiles that were not distinguishable from one another. But things are changing.
Fraternal (dizygotic) twins come from two eggs and two sperm and are as different as if born years apart. They are twins solely because they shared the mother's womb at the same time. But, identical twins (monozygotic) come from a single egg and sperm. They are formed when the fertilized egg undergoes its first division and the two new daughter cells move apart, each then proceeding to form a separate individual. Since they came from the same fertilized egg, the share the same DNA. In fact, the two would be indistinguishable by standard PCR-STR DNA Profiling.
But, in reality, even identical twins have distinct DNA. We just weren't able to see the differences. Before now.
As each twin embryo grows and develops in utero, and the cells continue to multiply, the replication (copying) of each twin's DNA isn't perfect. Minor errors or variations begin to appear so that by birth each Twin's DNA is slightly different from its sibling. And as life goes on, each twin is subjected to different environmental stresses, which is turn alters each one's DNA replication.
As opposed to STR, which looks at repeating short sequences of bases within the DNA strand, a newer DNA technique, known as Single Nucleotide Polymorphism (SNP), gives the examiner a complete DNA sequence of the strand being analyzed. That is the exact sequence of bases in each strand is determined and this can reveal the differences in the DNA of identical twins. Another newer technique known as High Resolution Melt Curve Analysis (HRMA) might offer still another method to make this distinction.
So even identical twins are not so identical.
Want to know more about DNA profiling? Check out the updated 2nd Edition of FORENSICS FOR DUMMIES.