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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?

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marianaegarciaperedo

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.    

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