Different versions of a gene, of which code for different versions of a characteristic.
If the alleles in a mtching pair are the same, they are called homozygous alleles.
If the alleles in a matching pair are different, they are called heterozygous alleles.
Some alleles are dominant to other forums of a gene and will always be expressed.
Some alleles are recessive to other forms of the gene and will rarely be expressed.
The physical expression of alleles which are pairs is the phenotype.