Solutions For An Introduction to Genetic Analysis Chapter 11 Problem 2P

Mutations in the gene encoding a repressor protein reveal that the protein is trans-acting, it an act on any copy of the target DNA (deoxyribonucleic acid) site in the cell.

Partial diploids were essential, for differentiating cis-acting from trans-acting mutants. This is definition one must introduce a second copy of the locus in trans to test the property of the mutants.