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A teenage girl allegedly discovered her dad was not her biological father after testing her blood type during a science lesson.

The girl, said to be studying at California State University , apparently announced to classmates the man she thought was her dad was in fact her UNCLE after her mum had an affair.

The shocking incident was publicised on Twitter by Anya Hettich, supposedly a fellow student at the US university.

According to Anya, the girl questioned her professor in a biology lecture after her blood type did not match those of her parents.

(Image: anyahettich/Twitter)

She allegedly told the lecturer her dad's blood was O and her mum's was A, yet hers was AB.

The professor told her the blood types were impossible and urged her to talk to her parents to double check her information was correct, Anya tweeted.

And when the girl returned to class after the lecture, she reportedly told 243 students her mum had an affair with her husband's step-brother, who fathered her.

Anya later tweeted the couple were getting a divorce.

(Image: anyahettich/Twitter)

In a lengthy thread, Anya tweeted: "So on Monday we’re discussing blood types and this girl was trying to figure out why her blood type didn’t make sense on her Punnett square

"She told the prof her dad was O and her mom was A but yet she was AB

"My prof explained that that’s impossible, that she’s confused on the blood types or something - she even drew out the different Punnett squares to show her

"The girl was persistent that she wasn’t wrong on the blood types and was confident that there was something wrong with how the prof was doing it

"SOOOO, she comes back to class today and my prof made a point to ask if she was mistaken or confused

"This girl is ballsy, lemme tell ya. She announces it to the entire class of 243 people

"TURNS OUT, her mom had an affair with her husbands brother which is his step brother

"So technically, she’s been raised by her uncle and not her dad her entire life - her mom hid it from the both of them for 21 years

"Because my prof told her she was wrong with her blood types, she figured out her uncle was actually her dad and vice versa

"Now they're getting a divorce."

The girl has not spoken publicly about the alleged discovery.