 A tryptophan auxotroph cannot make tryptophan for itself, but wild-type cells can and so a revertant will revert to the normal state of being able to produce tryptophan.He found that when yeast colonies were moved from a tryptophan-rich medium to a minimal one, revertants continued to appear for several days.The degree to which revertants were observed in yeast was not as high as with bacteria.