To study the mechanisms of partial dominance, 12 intragenic alcohol dehydrogenase null mutations (Adhn) in Drosophila melanogaster were used as a model system. These known sequenced mutations, either single base substitutions or small intragenic deletions (9-16 bp) were analyzed for peptide production, dimer formation and enzymatic activity of the heterodimer. Multiple mechanisms leading to partial enzyme expression in heterozygotes and a wide range of dominance varying from almost complete recessive to a high degree of dominance were found. An expression of partial dominance for alcohol tolerance was observed for all 12 Adhn under the high stress of 10% ethanol. The genetic background on which the null mutations are expressed is a major determinant for alcohol tolerance of the heterozygous adult flies. The mutations induced by X-ray had higher average dominance than that of ENU-induced mutations, and the mutations that formed nonfunctional heterodimers had the highest dominance.