Gene S controls the sharpness of spines in a type of cactus. Cactuses with the dominant allele, S, have sharp spines, whereas homozygous recessivess cactuses have dull spines. At the same time, a second gene, N, determines whether or not cactuses have spines. Homozygous recessive nn cactuses have no spines at all. A cross between a true-breeding sharp-spined cactus and a dull spineless cactus would produce ___________.
a. all spineless progeny
b. all sharp-spined progeny
c. 50% sharp-spined, 50% dull-spined progeny
d. 25% sharp-spined, 50% dull-spined, 25% spineless progeny
e. It is impossible to determine the phenotypes of the progeny