Homozygous

having identical genes at one or more loci

Description
Humans are diploid, meaning that they possess a duplicate set of chromosomes and hence two copies (a paternal and a maternal one) of most genes. Being homozygous (homozygosity) for a given gene means possessing two identical copies of that gene. Homozygotes for certain disease traits, who therefore do not have a functional copy of the gene in question, may be severely affected by disease symptoms (see e.g. thalassemia and sickle cell disease). See heterozygous