A method for deciding whether two mutations having similar effects occur in the same or different genes (defined as units of function). If the mutations occur in the same gene, only heterozygotes with the *cis arrangement
will show the normal phenotype because the gene in the homologous chromosome is dominant wild type and codes for normal protein. In the *trans arrangement
both copies of the gene contain a mutation and thus there is no wild-type copy of the gene and hence no normal protein. If the mutations occur in different genes, specifying different polypeptides, all heterozygotes will be normal in appearance because the homologous chromosomes carry wild-type copies of the genes.