Theory
In this scene in season 4 episode 7, Henry said that Dr. Brenner wanted to recreate him. He also said El was born after that.
Also remember in season 2, El found out Terry Ives(her mom)was also a test subject of Dr. Brenner's.
Dr. Brenner probably took Henry's DNA and Terry's DNA and created Eleven. It explains why only Henry and El have the same powers. The other test subjects have different types of powers,they are weaker.
If you google 'Who is El's father?' it gives you the name - Andrew Rich. Andrew is not a character on the TV show,which could possibly cancel him out of the options,this is just my theory. If my theory's kinda of correct,I can't wait to find out how it will revealed in season 5. A fight between father and daughter.
💡What do you guys think?💡