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 and Explanation:

Available data:

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

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

        SsNn

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

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

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