The presence of two different mutant alleles at a particular gene locus, one on each chromosome of a pair. The human genome contains two copies of each gene, a paternal and a maternal allele. A mutation affecting only one allele is called heterozygous. A homozygous mutation is the presence of the identical mutation on both alleles of a specific gene. However, when both alleles of a gene harbor mutations, but the mutations are different, these mutations are called compound heterozygous. Also called a genetic compound.