Unit of heredity (4)
I believe the answer is:
gene
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unit of heredity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(heredity means passing on genetic characteristics)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for gene that I've seen before include "Something got from parent < for boy" , "The Selfish -- (Dawkins)" , "Biological factor determining heredity" , "DNA unit" , "Hereditary determinant" .)