linux is not a clone of unix.
linux is closer to a clone of mimix. -not that that matters.
Linux is a unix "like" system, in so far as the way it manages hardware and memory is simillar, and a lot of the utilities, (copy move, cd) etc were re-written and released under a GPL license.
those tools are called GNU. (Recurring achronym meaning GNU's Not Unix
If you find a tool that you find especially useful in Linux, (and the GNU tools are especially useful for command line work) there are ports to the windows system at GNUwin32 project.
anyway... the short answer to your question is...
There are a few different versions of Unix.
but UNIX was invented by Bell Labs. it's been bought and sold by various people, re-invented as well... but it still remains a commercial and patent protected piece of software. Linux on the other hand does not infringe on any of those patents.
it's completely written from fresh...
Apple desktops are Unix machines.