The Collaborative International Dictionary of English v.0.48:

zygosity \zy*gos"i*ty\ (z[-i]*g[o^]s"[i^]*t[y^]), n. (Biol.)
   The property of being either homogeneous or heterogeneous
   with respect to the allelic genes determining some hereditary
   trait in a diploid or polyploid organism; if homogeneous, the
   organism is homozygous; if heterogeneous, it is heterozygous.
   [PJC]