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 percent sharp-spined, 50 percent dull-spined progeny
C) 25 percent sharp-spined, 50 percent dull-spined, 25 percent spineless progeny
D) all spineless progeny
E) It is impossible to determine the phenotypes of the progeny.

Question

Quizplus · Accepted Answer

The true-breeding sharp-spined cactus must have the genotype SSNN (homozygous dominant for sharp spines and having spines), and the spineless cactus must be nn for the N gene (homozygous recessive for no spines). Since the spineless cactus can only contribute the n allele and the sharp-spined cactus can only contribute the S and N alleles, all progeny will have at least one S and one N allele, resulting in all sharp-spined progeny.

Gene S Controls the Sharpness of Spines in a Type