Polar body twinning occurs when a egg splits prior being fertilized, and each half is fertilized by a different sperm.

While they are classified as fraternal twins, they share 75% of their DNA (They share 100% of the DNA from their mother, and at least 50% of the DNA from their father). Because of this they look more alike than do most fraternal twins.

This is sometimes called 'half-identical twins'.

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