Our genetic code retains evidence of mistakes written in the past. A code employing both Cyt and Ura (to which Cyt can decay) is ridiculous. So is using Thy as a 'solution'
Taken from my Spring 2014 MCB184 class; references appear in the Powerpoint/Keynote in the next image.These ideas build on preceding Blue eyes material
Lecture #1
Lecture #2
References embedded