Genetic expression of aryl hydrocarbon hydroxylase activity in the mouse: Distinction between the “responsive” homozygote and heterozygote at the Ah locus

β-Napththoflavone administration induces certain monooxygenase activities, such as aryl hydrocarbon (benzo[ a]pyrene) hydroxylase, and cytochrome P 1-450 formation in the “responsive” C57BL/6 and C3H/He inbred mouse strains, whereas these changes are absent or relatively small in the so-called nonresponsive DBA 2 inbred strain. Dose-response curves—with the use of large numbers of animals of the same age and sex and with either β-naphthoflavone or the much more potent 2,3,7,8-tetrachlorodibenzo- p-dioxin as inducer—reveal a small, but statistically significant, difference in the hydroxylase induction between the C57 BL 6 J homozygote and the ( C57 BL 6 J )( DBA 2 J )F 1 heterozygote in liver, kidney, bowel, and lung. The (C3H/HeJ)( DBA 2 J )F 1 heterozygote displays additive inheritance in each of these same tissues.