The human Y-chromosome is small and contains fewer genes than the X-chromosome. Therefore, there are very few Y-linked disorders. Retinitis pigmentosa is one of the Y-linked disorders. Y-linked disorders are uncommon in the human race for a number of reasons, one being that only males have a Y chromosome (females have two X chromosomes), a second reason is that the disease must be dominant; humans only have one Y chromosome - there will not be another copy of the recessive gene to make the disorder apparent in the human. There are, however, some disorders that lead to multiple Y chromosomes being present in the genotype: Y linked disorders will be more common in these people.