The most common and most likely answer to this crossword clue is the 5 letter word GENIC
I've seen this crossword clue in The Daily Telegraph
G
E
N
I
C
More clues leading to the result GENIC
Buddy explains!
The straight part
We know that "it’s to do with units of heredity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s to do with units of heredity = genic.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does GENIC mean?
GENIC adjective- Of, relating to, produced by, or being a gene