What 20th century discovery has been instrumental to the field of genetic engineering by allowing researchers to cut and splice specific dna fragments into microorganisms?

Gene editing.

Gene editing, is based  on a the CRISPR-Cas9 technology. It allows researchers to customize a living organism’s genetic sequence by making very specific changes to its DNA. Gene editing has a wide array of applications, being used for the genetic modification of crop plants and animals.

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.