Histocompatibility, or tissue compatibility, is the property of having the same, or mostly the same, alleles of a set of genes called human leukocyte antigens (HLA). Each HLA allele represents a distinct immunological tissue type. On a populational level there is a great number of different alleles at each HLA loci on Chromosome 6 at 6p21.3 in humans. However on an individual level a person can only inherit two alleles for each HLA gene locus due to the limits of sexually reproductive meiosis. This creates a situation where a person's spread of HLA alleles (or tissue types) may or may not match another's. Histocompatibility is thus a measure of how similarly two people's HLA alleles or tissue types matches up. The more closely two people's HLA alleles match up, the less one's donor tissue graft will be rejected by the recipient's immune system.