Proto-Indo-European ǵenu- "knee, angle" (homophonic with ǵenu- "jaw") in the form ǵneu- became Proto-Germanic *knewam, Old English cnēo, English knee.
The suffixed variant *ǵōnw-yā- (*ǵōnw-yeh₂?) became Greek γωνία gōnia "point, edge, angle", borrowed as the -gon of polygon. (poly- is from πολύς "many" from PIE *pelh₁- "to fill".)
In Latin the root became genū "knee", as in genuflect. The ODEE states that genuine is from Latin genuīnus "innate, natural" from genū: "The orig. ref. was to the recognition of a new-born child by a father placing it on his knees". Alternately, genuīnus is an alternation of ingenuus "native, indigenous, not foreign" (here), which is from *ǵenh₁- "to give birth, beget".