In an individual belonging to a sexual and diploid species, each gene exists in two copies, one inherited from the father and the other from the mother.
Each gene can exist in one or more variants, called alleles. A gene coding for the color of the eyes can thus exist in several variants coding several colors or a gene encoding a protein can exist in several variants, coding slightly different forms of the protein.
Homozygote: gene represented by two identical alleles. By extension we speak of homozygous individual for this gene.
In genetics, we speak of pure strain for a breeding population identical to itself, usually a population where each individual has the same genotype and where each gene is homozygous.
HOMOZYGOTIE is opposed to HETÉROZYGOTIE which consists in having for the same gene two different variants.
An organism is heterozygous for a gene when it has two different alleles of that gene, one locus for each of its homologous chromosomes.