In both the core D&D Gamma World game and the Famine in Far-Go expansion, all origins (with the exception of Engineered Human) have the same chance to be rolled during character creation.
The defaut random origin generation usually leads to a sort of de facto balance, but it’s still possible to create a character without usable at-wills (or even basic attacks). With the introduction of the origins in Famine in Far-Go it’s possible to go to the other extreme, and create an overpowered character (say, Magnetic/Temporal) that could dominate any combat with ease.
A number of suggestions have been proposed to fix this imbalance, including powering up the weaker origins by making their Novice powers at-wills instead, and by restricting the powers and traits of the stronger origins.
The system presented in this document offers another approach, based on the concept of rarity used in collectable card games such as Magic: the Gathering – or in D&D Gamma World itself, with the booster packs.
Each origin is rated as Common, Uncommon, or Rare based upon the relative power level of that origin. Note that rarity is used as a metaphor here, and not literally; Uncommon origins may be more common than Common origins in many games.
The general principle is to minimize the chance of random under- or overpowering by restricting the combination of origins possible.