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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.
-If doubly heterozygous SsNn cactuses were allowed to self-pollinate, the F₂ would segregate in which of the following ratios?
A) 3 sharp-spined:1 spineless
B) 1 sharp-spined:2 dull-spined:1 spineless
C) 1 sharp-spined:1 dull-spined:1 spineless
D) 1 sharp-spined:1 dull-spined
E) 9 sharp-spined:3 dull-spined:4 spineless

Question

Quizplus · Accepted Answer

The correct segregation ratio for the F2 generation from a doubly heterozygous SsNn self-pollination, considering the independent assortment of genes, is 9 sharp-spined:3 dull-spined:4 spineless. This follows from the Punnett square analysis of a dihybrid cross, where the phenotypic ratio for two independently assorting genes each with a dominant and recessive allele is 9:3:3:1, modified here by the fact that the presence of spines is a prerequisite for sharpness or dullness to be expressed.

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