Noncestor: N, one whose attributes make it clear that a particular person is not descended therefrom and shares no DNA therewith; not a progenitor.
Uncestor: N, one whose attributes make it impossible that a particular person is descended therefrom or shares any DNA therewith; extremely NOT a progenitor.
"I've been reading about President Obama's family tree.
His noncestors include Calvin Coolidge and Grover Cleveland.
Washington and Theodore Roosevelt are on the uncestor page."
(Origin: 2009. Coined and first lexicographic publication by Staghounds.)