Trait origin (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
gene
'trait origin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gene that I've seen before include "Part of chromosome" , "Hereditary DNA sequence" , "Unit of heredity significance" , "Hereditary molecule" , "___ Kelly, star of Singin' In The Rain, d. 1996" .)