Does a father get כרת  if he does not do מילת בנו.  (רמב"ם) .

Is the chiyuv of the father his own חיוב, or is he responsible for his sons חיוב. (ברכת שמואל).

Listen to how the תרץ of תוס' גיטין has monumental consequence to these נידונים.

Enjoy!!

E.C.- See if you can be מכוון  to one of the other eight תירוצים given by the meforshim on this תוס'.