Facts on key bit of inheritance (4)
I believe the answer is:
gene
'bit of inheritance' is the definition.
(I've seen this in another clue)
'facts on key' is the wordplay.
'facts' becomes 'gen'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'key' becomes 'e' (musical key).
'gen'+'e'='GENE'
(Other definitions for gene that I've seen before include "Hereditary determinant" , "Bodily factor controlling heredity" , "may be passed to child" , "Element of heredity" , "Hereditary unit on chromosome" .)