  The continuation of resonant periodic orbits from the restricted to the
general three body problem is studied in a systematic way. Starting from the
Keplerian unperturbed system we obtain the resonant families of the circular
restricted problem. Then we find all the families of the resonant elliptic
restricted three body problem which bifurcate from the circular model. All
these families are continued to the general three body problem, and in this way
we can obtain a global picture of all the families of periodic orbits of a
two-planet resonant system. We consider planar motion only. We show that the
continuation follows a scheme proposed by Bozis and Hadjidemetriou (1976) for
symmetric orbits. Our study includes also asymmetric periodic orbits, which
exist in cases of external resonances. The families formed by passing from the
restricted to the general problem are continued within the framework of the
general problem by varying the planetary mass ratio $\rho$. We obtain
bifurcations which are caused either due to collisions of the families in the
space of initial conditions or due to the vanishing of bifurcation points. Our
study refers to the whole range of planetary mass ratio values ($\rho \in
(0,\infty)$) and, therefore we include the passage from external to internal
resonances. In the present work, our numerical study includes the case of the
2/1 and 1/2 resonance. The same method can be used to study all other planetary
resonances.
