Hereditary unit (4)
I believe the answer is:
gene
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hereditary unit</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for gene that I've seen before include "Hereditary determinant" , "Factor controlling heredity carried by chromosome" , "What makes us different" , "Something got from parent < for boy" , "Part of chromosome" .)