Book is on units of heredity (7)
I believe the answer is:
genesis
'book' is the definition.
(first book of the Bible)
'is on units of heredity' is the wordplay.
'on' says to put letters next to each other.
'units of heredity' becomes 'genes' (I've seen this in another clue).
'is' put after 'genes' is 'GENESIS'.
(Other definitions for genesis that I've seen before include "Bible's first book" , "religious book?" , "OT opening" , "Beginning the holy book" , "Old Testament Book 1" .)