  Rotational bands with (neutron h_11/2)^1 (proton h_11/2)^1 configurations are
studied using a particle-rotor model in which a proton and a neutron
quasiparticles interacting through a zero-range force are coupled with a
triaxial rotor. It is shown for 124Cs that one can reproduce the signature
dependence of energy and B(M1)/B(E2) ratio best when one takes into account
gamma-deformations with irrotational-flow moment of inertia in addition to the
proton-neutron interaction proposed by Semmes and Ragnarsson. Including both
effects, a systematic calculation of signature splittings is performed for Cs,
La isotopes and N=75 isotones to be compared with experiments. Discussions are
also done on the deficiencies of the cranking model concerning its
applicability to signature inversion phenomena in odd-odd nuclei.
