Cross yy and yy. what percentage of the offspring will be purebred dominant?

Well if you do the box you will get;

25% of YY

50% of Yy and

25% of yy

I don't know which one is purebred dominant so if you know let me know. But this should help you.

Have a great day!

So if you consider a punnet square of two heterozygous parents Yy x Yy

       Y    y

Y     YY  Yy

y      Yy    yy

Now to break that down a little bit: a purebred offspring would have to be either homozygous dominant or recessive and that would either be YY or yy.

Since you know that a y is a recessive gene and a Y is a dominant gene you can look back to the punnet square and see

YY -25% (homozygous/purebred dominant)

Yy- 50% (heterozygous)

and yy- 25% (homozygous/purebred recessive)