Question

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?

Answer 1

Answer and Explanation:

Available data:

• S allele expresses sharp spines, and it is dominant

• s allele expresses dull spines, and it is recessive

• N allele expresses the presence of spines, and it is dominant

• n allele expresses the absence of the spines, and it is recessive

Cross: Doubly heterozygous SsNn individuals allowed to self-pollinate

Parental) SsNn x SsNn

Gametes) SN sN Sn sn / SN sN Sn sn

Punnet square) SN Sn sN sn

SN SSNN SSNn SsNN SsNn

Sn SSNn SSnn SsNn Ssnn

sN SsNN SsNn ssNN ssNn

sn SsNn Ssnn ssNn ssnn

• 9/16 individuals with sharped spines, S-N-

Their genotypic ratios are 1/16 SSNN, 2/16 SSNn, 2/16 SsNN, 4/16

SsNn

• 3/16 individuals with dull spines, ssN-

Their genotypic ratios are 1/16 ssNN and 2/16 ssNn

• 4/16 individuals with no spines at all, --nn

Their genotypic ratios are 1/16 SSnn, 2/16 Ssnn, and 1/16 ssnn.