Heredity unit (4)
I believe the answer is:
gene
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">heredity unit</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for gene that I've seen before include "- - Kelly, dancer" , "Bodily factor controlling heredity" , "___ Kelly, star of Singin' In The Rain, d. 1996" , "may be passed to child" , "Biological factor determining heredity" .)