Patty had split with Desi Arnaz Jr., who was only 17 at the time, and then married Michaell Tell, a man she barely knew. The marriage lasted just days, and the following year, the most likely father of Sean became known.
It emerged that Patty was having an affair with actor John Astin. The actress assumed John was Sean’s father. After Patty and John married, Sean’s last name was changed from Duke to Astin.
Desi didn’t try to deny it, so he and Sean started to build a relationship and soon became like family. And it wasn’t until Sean turned 26 that he learned that neither Desi nor John was genetically related to him.
Michael Tell’s relative contacted Sean (who, by the way, followed in his mom’s footsteps and became an actor). The relative claimed that Sean was Michael’s son.
To put unsubstantiated claims to rest, Sean had a DNA test which yielded surprising results: Michael really was his biological father. However, the test results didn’t damage Sean’s relationship with the man who raised him. Sean told People back in 2001:
And, although they now knew they were not biologically related, Sean and Desi remained friendly.
What did Patty say?
Patty did not encourage Sean to have that DNA test and refused to acknowledge its result. She continued to insist that Sean was Desi’s child.
Despite her denials, Sean wasn’t mad at his mom. After her passing in 2016, Sean spoke to Entertainment Tonight and praised his mom, calling her a 'very strong woman' and a 'warrior'.