  The Type-I seesaw model is a common extension to the Standard Model that
describes neutrino masses. The Type-I seesaw introduces heavy right-handed
neutrinos with Majorana mass that transform as Standard Model electroweak gauge
singlets. We initially study a case with two right-handed neutrinos called the
3-2 model. At an energy scale above the right-handed neutrinos, the effective
neutrino mass matrix is rank degenerate implying the lightest neutrino is
massless. After considering renormalization effects below the two right-handed
neutrinos, the effective neutrino mass matrix remains rank degenerate. Next, we
study a model with three right-handed neutrinos called the 3-3 model. Above the
energy scale of the three right-handed neutrinos, we construct the effective
neutrino mass matrix to be rank degenerate. After solving for the
renormalization effects to energies below the three right-handed neutrinos, we
find the rank of the effective neutrino mass matrix depends on the kernel
solutions of the renormalization group equations. We prove for the simplest
kernel solutions the effective neutrino mass matrix remains rank degenerate.
