Four sets of random propylene-based copolymers with 1-10 mol % of ethylene, 1-butene, 1-hexene, or 1-octene as co-units, synthesized with the same metallocene catalyst, were investigated by differential scanning calorimetry and wide-angle X-ray scattering following rapid and isothermal crystallization. Parameters related to defect concentration, defect type, and microstructure and thermodynamic and kinetic factors were evaluated as to their role in developing the gamma polymorph. The effect of the comonomer in enhancing the fractional content of the gamma polymorph is akin to the role of defects in the homo-poly(propylene) chain. However, differences in the partitioning of the comonomer between the crystalline and noncrystalline regions leads to contents of the gamma phase that differ among the copolymers at any given crystallization temperature. Qualitatively, these differences can be used to assess the degree to which a counit participates in the crystallite. The experimental results suggest that there is no discrimination of the defects that enter the crystal lattice (stereo, regio, ethylene, or butylene units) between the alpha or gamma crystallites. The results with copolymers establish that the bases that lead to the formation of the gamma polymorph are the same for homo-poly(propylene) and its copolymers.