The most common and most likely answer to this crossword clue is the 5 letter word GENES
I've seen this crossword clue in The Times
G
E
N
E
S
Buddy explains!
The straight part
We know that "they're passed on by parents" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they're passed on by parents = genes.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 GENES mean?
GENES noun- A theoretical unit of heredity of living organisms; a gene may take several values and in principle predetermines a precise trait of an organism's form (phenotype), such as hair color.
- A segment of DNA or RNA from a cell's or an organism's genome, that may take several forms and thus parameterizes a phenomenon, in general the structure of a protein; locus.