Alleles can be described as
i. dominant
ii. recessive
iii. codominant
iv. partially dominant
v. incompletely dominant

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.