I was actually prompted to think about this in the odd "Reid disrespects McDermott" thread yesterday. I started thinking about the Reid tree, which is really the Holmgren tree, which is really the Bill Walsh tree.... and then I went down the rabbit hole and.... OH WOW! We are living in a Bill Walsh world.
Of the 31 NFL Head Coaches currently employed only one - Mike Vrabel of the Patriots - has no link to the Bill Walsh tree. There was a time when the Walsh tree and the Parcells tree were seen as somewhat equal and while it is true that there are guys on the Walsh tree who also feature on the Parcells tree: Payton, Daboll, Campbell (and his guys) you can't get to anything like 30/31 for Parcells. This league is now fully under the influence of that lineage. Talk about leaving a legacy!
Anyway, the image below is my visual representation of it (apologies for image quality it wouldn't upload in higher res, will try and fix)
Link to clearer version of image