High-school biology reminder: We inherit one copy of the arginine vasopressin receptor gene from each of our parents. When the two copies are identical, we are homozygous for that gene. When the two copies of the gene represent different variants, or alleles, we are heterozygous. So in this study, a subject might have no genes with the 334 allele, or might be heterozygous for 334 and carry one copy, or might be homozygous and have two copies of 334.