Help Please!! Chi - Square for Biology?

For my lab practical I'm going to have to do a Chi Square. I understand how to do the calculations, but how do I get the PERCENTAGE? I'm confused as to how calculate expected value. So, for example, Joe planted 150 plants, he got 95 yellow seeds and 55 green seeds. He thinks that the cross that produced these plants is Gg x Gg cross. So, how do I know the percentages to calculate expected value and everything else?