Homozygosity: The state of possessing two identical forms of a particular gene, one inherited from each parent.
For example, a girl who is homozygous for cystic fibrosis (CF) received the CF gene from both of her parents and therefore she has CF.
The opposite of homozygous is heterozygous, the possession of two different forms of a particular gene, one inherited from each parent. For example, the father and mother of a CF child are heterozygotes for cystic fibrosis (CF). Each carries one normal gene and one CF gene but has no signs or symptoms of the disease.