Sex-linked genes are genes that are located on the sex chromosomes, X or Y, and their expression and inheritance patterns differ between males and females. In humans, the term sex-linked often refers to traits or disorders influenced by genes on the X chromosome, as it contains many more genes than the smaller Y chromosome. Males, who have only a single copy of the X chromosome, are more likely to be affected by a sex-linked disorder than females, who have two copies. In females, the presence of a second, non-mutated copy may cause different, milder, or no symptoms of a sex-linked disorder. Some of the more familiar sex-linked traits are hemophilia, red-green color blindness, congenital night blindness, some high blood pressure genes, Duchenne muscular dystrophy, and Fragile X syndrome. Sex-linked recessive diseases most often occur in males because they have only one X chromosome, and a single recessive gene on that X chromosome will cause the disease. Females can be carriers of X-linked recessive diseases and pass them on to their children.