Characters in The Big Bang Theory have some great back stories, including aspiring actor Penny (played by Kaley Cuoco). In the series Penny is the girl next door to Sheldon Cooper (Jim Parsons) and Leonard Hofstadter (Johnny Galecki). Later on in the series Penny and Leonard marry after realising their true feelings for one another.
However fans have pointed out a number of plot holes surrounding Penny’s character.
She is renowned for name-dropping her family members, one being her sister, Lisa.
Viewers have picked up on the fact they never meet Lisa in the show, but she is talked about a lot throughout.
Watchmojo.com said: “Here’s what we do know about Lisa: Her water broke at her own wedding, she shot her husband when they were both drunk and she gave Penny a bikini wax using crayons when they were younger.
READ MORE: Big Bang Theory: Did you spot glaring Howard plot hole?
Who is Penny's sister in The Big Bang Theory? (Image: Channel 4)
Big Bang Theory: Why was Penny's sister not featured? (Image: Channel 4)
Who is Kaley Cuoco? (Image: Getty Images)