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QUINCY, IL – These adorable baby girls are Kalani and Jarani Dean. Despite their different appearances — Kalani has light skin and blue eyes, and Jarani has dark skin and brown eyes — they are actually twins.

The girls’ parents are Whitney Meyer and Tomas Dean, from Quincy, Illinois. Meyer has been posting pictures of the unique pair since they were born last April.

The couple obviously couldn’t believe their luck.

“At first when they were born, I wanted to believe it but it’s so rare I didn’t think it’d happen to my twins!” Meyer told KHQA-TV. “But sure enough they’re biracial twins!”

Meyer is white and Dean is black, which explains part of this phenomenon. The twins are dizygotic, which is just the scientific term for fraternal twins. So each baby girl started out as a separate fertilized egg (monozygotic, or identical twins, start off from the same egg and then divide).

In a fraternal twin situation, each baby inherits a different combination of genes from mom and dad, so when the parents are of different races, it is possible the inherited genes for skin color can differ.

Still, the striking occurrence is rare — in Britain, for instance, the rate of mixed twins has been estimated at about 1 in 500 for mixed-race couples.