Although heredity lays down the of who we are, environment build who we will become.

Traits, genetic information

Explanation: we have the sentence:

"Although heredity lays down the of who we are, the environment helps build who we will become."

Heredity is the passing of traits (genetics) from the parents to their offsprings.

So the first part of the sentence can be completed as:

"Although heredity lays down the traits of who we are,..."

This also can be completed with something like "genetic information" if you want to be more "scientific" abut it

and we get in this case:

"Although heredity lays down the genetic information of who we are,..."

because growing process had stoped after 4-5 year

what is the actual question though?