Anomalous Generation Numbers in SO(10) and Supersymmetric SO(10)   Unification Theories
  One of the interesting features in unification models and supersymmetric unification models is that the chiral states of quarks and leptons in a family including a right-handed neutrino can be fitted neatly into a fundamental spinor representation (f.s) of dimension 16 for the SO(10) gauge group. However, it is shown in this paper that such a fundamental spinor representation of SO(10) for Weyl fermions will generate global (non-perturbative) gauge anomalies (of new type) when restricting to the $SU(2)\otimes SU(2)\otimes SU(2) \otimes SU(2)$ gauge subgroup. Such an example is the four SU(2) factors obtained through the reduction of the subgroup $SU(2)\otimes SO(7)$ of SO(10) with the SO(7) to the three SU(2) factors. The branching rule in this case is given by $(f.s)\rightarrow (2-1-2-2)\oplus (2-2-1-2)$ in terms of dimensions. A consistent gauge theory implies the gauge symmetry in a gauge subgroup, and then needs to be well-defined when restricting to the gauge subgroup. Consequently, a consistent SO(10) quantum theory needs to satisfy our selection rule $N_f+N_{mf}=even\geq 4$, namely the total number of generations with $N_f$ ordinary fermion families and $N_{mf}$ mirror fermion families is even and larger than three, and the three generations of chiral fermions in this content can not correspond to a consistent theory. Then we expect that there exist at least one additional fermion family including a right-handed neutrino or at least one family of mirror fermions including a left-handed mirror neutrino if SO(10) unification theory is relevant to our realistic world. The next possibility is to have total six generations that may subject to stringent 
Huazhong Zhang