If an individual carries a pair of alleles that are the same, they are homozygous.
Because humans are diploid, they have two matching sets of chromosomes and the same locus gene on each of their two sets of homologous chromosomes (exception in males, because they have XY chromosomes). If alleles (a variant form of a given gene) are the same, the organism is homozygous, otherwise, it is heterozygous for that loci.
