zy·gos·i·ty (z?-g?s??-t?)n. The genetic condition of a zygote, especially with respect to its being a homozygote or a heterozygote.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.zygosity (za????s?t?) n (Genetics) genetics the similarity or differences between DNA sequences on the chromosomes of a fertilized eggCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014zy?gos?i?ty (za??g?s ? ti, z?-) n. 1. the characterization of a hereditary trait in an individual according to whether the gene pairs for the trait are homozygous or heterozygous. 2. the characterization of twins, triplets, etc., according to whether they are monozygotic or dizygotic. [1945?50; probably derivative of -zygous (see heterozygous), on the model of viscous: viscosity, etc.] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.