Following up on his suspicion, de Cramer contacted nearby university scientists, who took blood from the twins. The blood tests confirmed that the two pups were genetically identical, the first such case. Later DNA testing of the puppies at six weeks old confirmed the confirmation.
de Cramer and the team have documented the discovery in a study in the journal Reproduction in Domestic Animals.
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What's unclear is how often identical twin puppies are born. New puppies often have similar physical characteristics, but they don't tend to have their DNA tested, and owners are unlikely to take a count of placentas vs. puppies in non-C-section births.
"It has taken so long for us to find a monozygotic [identical] pair, so they are probably rare," Carolynne Joone, of James Cooke University, told the BBC. "But so many of them will have been born naturally and blissfully unaware."
"We just happened to be lucky enough to be able to confirm it genetically," she said.
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