[Campbell Biology P.339] Concept Check 14.4 asks to compare the phenotypic ratio of a monohy... | Practice Question

Concept Check 14.4 asks to compare the phenotypic ratio of a monohybrid cross for flower color F2 gen with the offspring of the second-generation couple in the PTC tasting pedigree Fig 14.15b . What are the two ratios, and what accounts for the difference?

  • A: Both are 3:1; there is no difference.
  • B: 3:1 (dominant:recessive) and 1:2:1 (taster:taster:non-taster); difference is due to complete vs. incomplete dominance.
  • C: 3:1 (dominant:recessive) and 1:2:1 (homozygous dominant:heterozygous:homozygous recessive); difference is comparing phenotype ratio to genotype ratio.
  • D: 3:1 (dominant:recessive) and 3:1 (taster:non-taster); the difference is due to the specific parents in the pedigree cross (Tt x Tt) versus a general F1 cross.

Explanation

The flower cross F1 heterozygotes self-pollinating yields a 3:1 phenotypic ratio purple:white but a 1:2:1 genotypic ratio PP:Pp:pp . The PTC cross in the pedigree involves two known heterozygotes Tt x Tt . The question asks for the phenotypic ratio of their offspring, which is 3 tasters TT, Tt to 1 non-taster tt , or 3:1. The question in the book is tricky, it asks for the phenotypic ratio of the second generation couple in 14.15b, which is 1:2:1 TT or Tt : Tt : tt . The difference arises because one is a classic phenotypic ratio of complete dominance, while the other is a genotypic ratio which becomes the phenotypic ratio in cases of incomplete dominance or codominance.