A woman has revealed how a science experiment during a biology class led to her classmate finding out her dad wasn't her real father.

California State University student Anya Hettich, 19, from San Bernadino, California took to the micro-blogging site to share details about what happened and explained that during an 8am biology class, the students were discussing blood type.

She revealed that while discussing blood types, a classmate couldn't figure out why her blood type didn't make sense - her dad was blood type O while her mother was A yet she was AB.

Reveal: Twitter user @anyahettich from San Bernadino, California, has shared how her classmate found out her dad wasn't her real father during a biology class (stock image)

Confusion: The class was testing blood types and the girl couldn't figure out why her parents had O and A blood types but she was AB

Formula: To have an AB blood type, one parent must be A and one must be B

Scientific: Their professor explained that it was impossible for the girl's father to have an O blood type

Story teller: The moment was detailed online by classmate Anya Hettich (pictured left)

'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 wrote.

'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,' she added.

Blood type is based on the presence or absence of the A and B antigens on your red blood cells.

The A blood type has only the A antigen and the B blood type has only the B antigen.

The AB blood type has both A and B antigens, and the O blood type has neither A nor B antigen.

Each biological parent donates one of two ABO genes to their child.

The A and B genes are dominant and the O gene is recessive so if an O gene is paired with an A gene, the blood type will be A.

Disbelief: The Twitter thread revealed that the girl didn't want to believe the test was right

Burden of proof: However their professor told her to double check with her parents

Family secret: The girl shared with the class the next day that her mom had an affair with her dad's stepbrother so her uncle was really her father

Split: The girl also shared that her parents were now getting a divorce

Anya then revealed that the girl was 'persistent that she wasn’t wrong on the blood types' and was confident that there was something wrong with how their professor, Lisa Anderson was conducting the test.

However, as the scientific evidence suggested otherwise, Professor Anderson told the girl to go back to her parents and double check their blood types.

When the girl returned to class the next day, she announced to 'the entire class of 243 people', that her mom had an affair with her husband's brother who was actually 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,' she explained.

Anya concluded the thread by revealing that the girl's parents were now getting a divorce.