The given blank can be filled with forward genetics.
Forward genetic screen or forward genetics refers to a method used to determine a set of genes or genes accountable for a specific phenotype of a species. On the other hand, reverse genetics or reverse genetic screen evaluates the phenotype of a species following the disruption of a known gene.
The method of forward genetics was initially done by inducing mutants with radiation, by using naturally occurring mutations, insertional mutagenesis, or chemicals.