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

08-04-2016
Biology

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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

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The answer is E) 9 sharp-spined:3 dull-spined:4 spineless.

It is known that the in dihybrid cross (a cross between two different traits), the phenotypic ratio between the offspring in the F2 generation is 9:3:3:1:
9 - individuals with both dominant traits (SSNN),
3 - individuals with the first dominant trait and the second recessive trait (SSnn),
3 - individuals with the first recessive trait and the first recessive trait (ssNN),
1 - individual with both recessive traits (ssnn).

According to this, there will be 9 sharp-spined cactuses (SSNN), 3 dull-spined cactuses (ssNN), and 4 spineless cactuses (3 cactuses with SSnn genotype and 1 cactus with ssnn genotype).