  A non-minimal approximation for effective masses of light and heavy neutrinos
in the framework of a type-I seesaw mechanism with three generations of sterile
Majorana neutrinos which recover the symmetry between quarks and leptons is
considered. The main results are: (a) the next-order corrections to the
effective mass matrix of heavy neutrinos due to terms O({\theta}MD) are
obtained, which modify the commonly used representation for the effective mass
(MD is a Dirac neutrino mass when the electroweak symmetry is spontaneously
broken); and (b) the general form of the mixing matrix is found in non-minimal
approximation parametrized by a complex 3x3 matrix satisfying a nontrivial
constraint. Numerical analysis within the {\nu}MSM framework demonstrates the
very small effect of new contributions of direct collider observables as
opposed to their possible significance for cosmological models.
