In squirrels, the gene for gray fur (G) is dominant over the gene for black fur (g). If 50% of a large litter of squirrels are gray, the parental cross that produced this litter was most likely

Gg and gg will most likely be the parental cross.

This is because G is dominant over g, meaning that the cross of G dominating g will happen one in two of every offspring, in other words, meaning that 50% of the offspring will have grey fur.