Gene S Controls the Sharpness of Spines in a Type

question 21

Multiple Choice

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