I learned this from this video (Note: contains 2000 year old bronze phalli) — although it seems like 祖 used in the sense of penis/phallus is a bit of a formal, literary usage, which is seemingly unattested in Sino-Xenic, so this information won't be of much use to you if you're, say, studying Japanese rather than Chinese. So it's only in Chinese where 祖 can mean penis, and only as a very sort of delicate and academic desexualized term for it in compounds like {铜祖|tóngzǔ} ("bronze phallus"), which was the term used in the video. That's my understanding, at least.
It is interestingly enough apparently believed (as one theory among several) that the right half of the character 祖 may in fact originate from an old pictogram of a phallus, "the symbol of the male ancestor"; on the other hand, the radical on the left-hand side (radical 113) on its own already carries a meaning of "ancestor" or "veneration", coming from an old pictogram of an altar.
As for readings, Middle Chinese tsu^X^ bears Standard Chinese zǔ and Cantonese zou^2^. For other readings and other meanings of the character (which are also interesting!) you can check Wiktionary — I've regurgitated enough of that site in this post already.
In any case, "ancestor" and "penis" sharing a character in Chinese reminds me of how in Arabic (and proto-Semitic), "memory" and "penis" share the same triliteral root, ḏ-k-r — that triliteral root, incidentally, gives us the name Zechariah through the Hebrew for "the Lord has remembered", and the Islamic term dhikr through the Arabic for "remembrance".