Commonly speaking, many Unices are modifications of the
BSD kernel and
SysV from AT&T.
Linux is not -- it is written from scratch. As such, there are some areas where Linux has made some real innovations, and
some areas where Linux would have done better to have followed BSD's lead (i.e. networking).
This is why you occasionally have snobbery from OpenBSD and FreeBSD fanatics who will claim that they are the "true" Unix and Linux is an upstart invader. It is precisely this attitude which led to Linux's dominance, as the BSD development team wasn't willing to accept pieces of code mailed in from any 13 year old unknown hacker on the net.