This root was the basis for Elvish “son” words for much of Tolkien’s life. The earliest indications of this root are ᴱQ. †Y̯ó (or y̯ond-) “son” and ᴱQ. yondo “male descendant”, both tied to the patronymic suffix ᴱQ. -ion “son of, descendant of” appearing in many names (QL/106). In the contemporaneous Gnomish Lexicon there was also the related patronymic prefix G. go- (GL/40), which implies the existence of a root ✱ᴱ√YO(NO) since [[g|initial [j] usually became [g]]] in Gnomish. However, go- was deleted and changed to G. bo-, along with new Qenya forms ᴱQ. vô and ᴱQ. vondo (GL/23, 40), implying a change to a root ✱ᴱ√VO(NO).
In Early Noldorin Word-lists from the 1920s, ᴱN. gó “son” reappeared along with ᴱQ. ion and yondi (PE13/144). In The Etymologies of the 1930s the root appeared as ᴹ√YŌ or ᴹ√YON “son” with derivatives like ᴹQ. yondo/N. ionn “son” and patronymic -ion (Ety/YŌ). However, in notes labeled “Changes affecting Silmarillion nomenclature” from the late 1950s, Tolkien wrote “Delete entirely yondo = ‘son’! Very unsuitable” (PE17/43). This particular note was rejected when Tolkien changed √YON “wide, extensive” to √YAN (PE17/42). Other notes in the same bundle indicate Tolkien was still seeking a new word for son, saying “Q wanted: son, daughter”, though yon(do) remained among the forms he was considering (PE17/170, 190).
However, it seems Tolkien eventually stopped vacillating and restored √YON, since the patronymic -ion was never discarded, and yon- was the basis for “son” words in notes from the late 1960s (VT47/26).