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Semi-identical twins are the result of a very rare form of twinning in which the twins inherit exactly the same genes from their mother but different genes from their father. The exact mechanism of their conception is not well-understood.
This situation is not the same as the common form of fraternal twinning, in which two genetically different ova are fertilized by two genetically different sperm from the same man. In this case, the ova are genetically identical. Nor is it the same as when two genetically different ova are fertilized by sperm from two different men. That phenomenon is known as heteropaternal superfecundation and results in fraternal twins also being half-siblings.
Semi-identical twins would be more genetically similar than typical siblings or fraternal twins (who are only as similar as other siblings), but do not share their entire genome as identical twins do. On average, Semi-identical twins would share about 75% of their genome. whereas identical twins would share almost 100% of their genome while full siblings would share only about 50% of their genome.
Only proven case Edit
In 2007, a study reported a case of a pair of living twins, one a hermaphrodite and one a phenotypical male. The twins were both found to be chimeras and to share all of their maternal DNA but only half of their father's DNA.
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