Essential Genetics 6th Edition by Daniel Hartl

Exercise 14

The homeotic floral-identity genes X , Y , and Z in whirligigs act either separately or in pairs to control the identity of floral organs. The floral structure consists of four whorls, the outermost being whorl 1 and the innermost whorl 4. Gene product X is expressed in whorls 1 and 2, Y in 2 and 3, and Z in 3 and 4. The presence of X without Y induces sepals, X 1 Y together induce petals, Y 1 Z together induce stamens, and Z without Y induces carpels. In the absence of X, the domain of activity of Z expands to all four whorls. In the absence of Z, the domain of activity of X expands to all four whorls. Mutant alleles of X , Y , and Z eliminate characteristic floral organs from a specific whorl, and a different floral organ appears in its place. What would you expect for the phenotype of each of the following?
(a) A loss-of-function X allele
(b) A loss-of-function Y allele
(c) A loss-of-function Z allele