Polygenic inheritance Transitional Reasoning Practice

Questions

1. 1.

   In a flowering plant, corolla tube length is governed by three independently assorting genes (A, B, and C) that act additively, with each dominant allele contributing equally to the phenotype over a baseline length of 12 mm. A true-breeding strain with 12 mm corollas (aabbcc) is crossed with a true-breeding strain with 36 mm corollas (AABBCC), and the resulting F1 plants are self-pollinated to produce an F2 generation. Which of the following correctly identifies the corolla length of the F1 generation and the expected proportion of F2 progeny exhibiting this exact intermediate phenotype?

   • A) 24 mm, with an expected proportion of 20/64 because the phenotype is produced by any genotype possessing exactly three dominant alleles.
   • B) 24 mm, with an expected proportion of 8/64 because only the completely heterozygous genotype (AaBbCc) can produce this phenotype.
   • C) 18 mm, with an expected proportion of 20/64 because each dominant allele contributes an increment of only 1 mm over the base phenotype.
   • D) 24 mm, with an expected proportion of 1/64 because each distinct phenotypic category in a trihybrid cross occurs with equal frequency.

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