Q. What is Trans conductance?

It is ratio of small change in drain current to the corresponding change in the gate-to-source voltage for a constant gate-to-source voltage .it is also called forward Trans conductance or forward transmittance. 

              G_m = ΔI_D/ΔV_GS

The value of the trans conductance is expressed in terms of siemens (s) or mhos .the transfer characteristic of JFET is apart of parabola i.e., it is non linear . Therefore the value varies depending upon the location on the curve as set by gate-to-source voltage. It has a greater value near the top of the curve than that near the bottom.