A form of mixing matrix for three active and three sterile, conventional Majorana neutrinos is proposed. Its Majorana lefthanded part arises from the popular bimaximal mixing matrix for three active neutrinos that works satisfactorily in solar and atmospheric experiments if the LSND effect is ignored. One of three sterile neutrinos, effective in the Majorana righthanded and Dirac parts of the proposed mixing matrix, is responsible perturbatively for the possible LSND effect by inducing one of three extra neutrino mass states to exist actively. The corresponding form of neutrino mass matrix is derived. If all three extra neutrino mass states get vanishing masses, the neutrino mass matrix is dominated by its specific Majorana lefthanded part. Then, the observed qualitative difference between mixings of neutrinos and down quarks may be connected with this Majorana lefthanded dominance realized for neutrinos. If m²₁ \simeq m²₂ for two of three basic neutrino mass states, the sum rule sin ² 2\theta _sol +  sin ² 2 \theta _Chooz/2 +  sin² 2\theta _LSND = 1 holds in the two-flavor approximation (for each of three cases). Thus, the solar neutrino oscillation amplitude, not fully maximal, leaves some room for the LSND effect, depending on the magnitude of Chooz effect (not observed so far).

PACS numbers: 12.15.Ff, 14.60.Pq, 12.15.Hh