Epistasis Transitional Reasoning Download PDF

Questions

1. 1.

   In a certain plant species, petal color is determined by two unlinked genes, $A$ and $B$, operating in a sequential biochemical pathway. The dominant allele $A$ encodes a functional enzyme that converts a colorless precursor into a yellow intermediate, whereas the recessive genotype $aa$ blocks this step, yielding white petals. The dominant allele $B$ directs the conversion of the yellow intermediate into a purple pigment, while the recessive allele $b$ produces an inactive protein that leaves the intermediate unchanged. If two dihybrid purple-flowered plants ($AaBb$) are crossed, which phenotypic ratio is expected in the resulting progeny?

   • A) 9 purple : 7 white
   • B) 12 purple : 3 yellow : 1 white
   • C) 9 purple : 3 yellow : 4 white
   • D) 15 purple : 1 white