Pretty funny operatic re-wordings, and decent enough fill with no major trouble points. Never heard of ALLELE ( 31A: Gene arising through mutation), but remembered the crosswordsy ELIA (27D: Kazan of Hollywood), so that first L was eventually found. Didn't particularly like LEDON (37A: Enticed), but the somewhat amusing MOO (30D: Stock answer?) finally changed the i to an O. And I was almost stumped by EEO (38D: Job listing inits). I'd heard of Equal Opportunity Employers, but didn't know that Equal Employment Opportunities were also a thing. And OBOES are (41D: Heckelphone cousins)? Who knew?
Learned today that "baba" is some kind of Spanish cake, but it wasn't too hard to guess, as anyone who's ever seen Bugs Bunny knows the Barber of Seville. And speaking of cartoons, I know Boris Badenov from Rocky and Bullwinkle, but had never heard of Mussorgsky's Boris Godunov (or the 16th c. tsar), but, well, it seemed to make sense somehow.
Particularly enjoyed the clues for OHARE (50A: Site of some Chicago touchdowns), PIANO (20A: One of the Baldwins?), and, for some reason, FOUNDED (25D: Like St. Augustine, in 1565). I thought at first that that last one might be FlooDED, but no. Eventually, it all made sense.