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Gene S controls the sharpness of spines in a type of cactus. Cactuses with the dominant allele, S, have sharp spines, whereas homozygous recessive ss 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 spineless cactus would produce
A) all sharp-spined progeny.
B) 50% sharp-spined, 50% dull-spined progeny.
C) 25% sharp-spined, 50% dull-spined, 25% spineless progeny.
D) all spineless progeny.
E) It is impossible to determine the phenotypes of the progeny.

Question

Quizplus · Accepted Answer

Since the sharp-spined cactus is true-breeding, it must have the genotype SSNN, and the spineless cactus must be nn for the N gene. The cross would result in all progeny having at least one dominant allele for sharp spines (S) from the sharp-spined parent and one recessive allele for spines (n) from the spineless parent, making them all sharp-spined (S_n_).

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