Biological trait carrier (4)
I believe the answer is:
gene
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">biological trait carrier</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for gene that I've seen before include "What makes us different" , "Embarrassment" , "and 16 Down: Rockabilly singer" , "DNA unit" , "Part of DNA" .)