The sex-linked "Orange" locus, "O/o", determines whether a cat will produce eumelanin. In cats with orange fur, phaeomelanin (red pigment) completely replaces eumelanin (black or brown pigment). This gene is located on the X chromosome. The orange allele is O, and is codominant with non-orange, o. Males can typically only be orange or non-orange due to only having one X chromosome. Since females have two X chromosomes, they have two alleles of this gene. OO results in orange fur, oo results in black or brown fur, and Oo results in a tortoiseshell cat, in which some parts of the fur are orange and others areas non-orange. Male tortoiseshell cats are known to exist, but, as expected from the genetics involved, they are rare and often exhibit chromosomal abnormalities. In one study, less than a third of male calicos had a simple XXY Klinefelter's karyotype, slightly more than a third were complicated XXY mosaics, and about a third had no XXY component at all.