Mirror twins (also known as mirror image twins) are one of the most
fascinating types of twin pairings of the 100 million+ twins all around
the world. It's a interesting concept and refers to twins who, when
facing each other, appear as matching reflections.
The majority of twins are not the rare mirror image twins. Fraternal twins
win the title of most common. Also known as dizygotic or non-identical, fraternal twins
are created from the fertilization of two separate eggs by two separate
sperms. The resulting twins have half of the same DNA, the same as any
other sibling, and may or may not have similar physical features.
Identical twins are monozygotic, meaning they were created when a single egg fertilized by one sperm, then divides into two separate embryos. A subset of identical twins is mirror image twins. They are rare and account for about one fourth of all identical twins totaling about 5 million twins.
This particular type of twinning may occur because the fertilized egg separates later than usual. The split occurs between 7 to 12 days after fertilization and the two identical halves develop into separate individuals who are genetically identical. A fertilized egg that splits after 12 days would likely result in conjoined twins. Though scientist continue to study the phenomenon of multiple births and why some factors happen in the womb, much about the creation of this type of twinning is still unknown and the probability of having identical twins seems to be random.
Identical twins always have the same sex and blood type. Though a few cases of identical twins of the opposite sex being born have been documented, this occurrence is extremely rare and unlikely. The term mirror twins, refers to the fact that they have the same physical features but opposite asymmetric features. If one twin is right handed the other twin is left handed, a twin with a birthmark on his left check will have a mirror twin with a birthmark on his right check. They can even have hair that whorls in opposite directions and opposite teeth. In more extreme cases, twins even have mirrored internal organs, such as one with heart on the right and the co-twin with his heart on the left - even the skeletal features may be reversed.
After birth, examination of the placenta can sometimes be used to
determine if twins are fraternal or identical, since identical twins are
likely to share the same placenta. But, the only way to tell for sure
if a set of twins are fraternal or identical after birth (unless they're
"mono-mono") is usually through genetic testing.
A cheek swab can be taken of both twins and a genetic profile will
reveal whether or not the twins are a perfect match genetically.
In the case of mirror image twins however, DNA testing cannot distinguish between them. Instead, a look at their different features and characteristics would be telling. If they consistently fall opposite of each respective twin, then you're likely looking at a mirrored pair.
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