The Collaborative International Dictionary of English v.0.48:

Prepotency \Pre*po"ten*cy\, n. [L. praepotentia: cf. F.
   pr['e]potence.]
   [1913 Webster]
   1. The quality or condition of being prepotent; predominance.
      [Obs.] --Sir T. Browne.
      [1913 Webster]

   2. (Biol.) The capacity, on the part of one of the parents,
      as compared with the other, to transmit more than his or
      her own share of characteristics to their offspring.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prepotency
    n 1: the state of being predominant over others [syn:
         predominance, predomination, prepotency]

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "prepotency":
   accomplishment, amperage, armipotence, ascendance, ascendancy,
   authority, balance of power, beef, black power, brute force,
   charge, charisma, clout, cogence, cogency, compulsion, deanship,
   dint, dominance, dominancy, domination, dominion, drive, duress,
   effect, effectiveness, effectuality, eminent domain, energy,
   excellence, favor, flower power, force, force majeure,
   forcefulness, full blast, full force, greatness, incomparability,
   influence, inimitability, lead, main force, main strength,
   majority, mana, masterdom, might, might and main, mightiness,
   moxie, muscle power, one-upmanship, overlordship, pizzazz, poop,
   potence, potency, potentiality, power, power pack, power structure,
   power struggle, powerfulness, precedence, predominance,
   predominancy, predomination, preeminence, preponderance,
   preponderancy, prepotence, prerogative, prestige, primacy,
   principality, priority, privilege, productiveness, productivity,
   puissance, pull, punch, push, right-of-way, seniority, sinew,
   skill, sovereignty, steam, strength, strong arm, success,
   superiority, superpower, supremacy, suzerainship, suzerainty,
   transcendence, transcendency, upper hand, validity, vehemence,
   vigor, vim, virility, virtue, virtuosity, virulence, vitality,
   wattage, weight, whip hand