Gene S Controls the Sharpness of Spines in a Type

question 27

Multiple Choice

Gene S controls the sharpness of spines in a type of cactus. Cacti with the dominant allele, S, have sharp spines, whereas homozygous recessive ss cacti 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) It is impossible to determine the phenotypes of the progeny.