Alleles - Variation of the Same Gene
Every gene has a specific location (called a locus) on a specific DNA strand. In organisms that reproduce sexually, traits are determined by the combination of genes from two parents. Variations of the gene (from different parents) are called alleles.
Homozygous and Heterozygous Alleles
The terms homozygous and heterogeneous refer to different combination of alleles for a specific trait.