Yellow squash Y is dominant over white squash y. A cross between YY and yy occurs. What are the phenotypic percentages?

By crossing a yellow squash (YY)  with a white squash (yy) , the phenotypic percentage of the offspring is 100% yellow squash (Yy).

Explanation:

The crossing of two pure lines -with identical alleles for a trait- dominant and recessive produces a hybrid or heterozygous offspring, with different alleles for that trait.

In this case, the yellow squash is homozygous dominant, while the white squash is recessive, obviously homozygous.

YY  X  yy

Alleles      Y     Y

y             Yy    Yy

y             Yy    Yy

All the offspring will have a Yy genotype and a single yellow squash phenotype for 100% of them.