In genetics, the term "heterozygous" describes an allele with a differing gene pair; "homozygous" refers to an allele with identical genes. Alleles determine the expression of gene traits.
Male and female parents of diploid organisms each provide a set of chromosomes to their offspring. Two of the chromosomes are called homologous chromosomes because they carry identical genes. However, these genes may contain different alleles.
Heterozygous alleles contain differing genes within their DNA pairs. Homozygous alleles contain identical DNA pairs. These two terms refer to the gene's genotype, which makes up an organism's genetic identity. The prominence of the alleles determine whether the genes they carry are expressed physically.