Differences in identical twins ?
Although genetically identical, monozygotic twins can be different in one incredible way. Scientists now believe that identical twins fall into one of two categories, either mirror image twins, or carbon copy twins.
What are mirror image twins ?
Mirror-image twins occur only in identical twins. A definitive explanation for mirror imaging is not known, but some researchers believe that it is related to the timing of the egg splitting after fertilization. It is thought that if the egg splits later than usual at around 7 days or after, the original right half of the egg becomes one individual and the original left half becomes the other.
These twins will often have “mirror images” of their features, such as opposite hair whorls or opposite dominant hands, even mirror-image fingerprints!. Other identifying features, such as birthmarks may be transposed, on the right shoulder of one and the left shoulder of the other, etc. There is no specific test for determining if twins are mirror-image. The determination is made by observation only, but those with mirror image twins will often talk of their twins lying in mirrored positionsand exhibiting other mirroring behaviour.
What are carbon copy twins ?
Carbon copy twins are identical twins who do not exhibit mirror-image twin markers. They are identical in every way – ie both right-handed, both hair whorls in the same direction etc etc. The genetic markers seem to have been imprinted on the twin as in a carbon copy.