Sex-linked inheritance is the inheritance of traits that are controlled by genes located on the sex chromosomes, which are the X and Y chromosomes in humans. Because females have two X chromosomes (XX) and males have one X and one Y chromosome (XY), sex-linked traits can be inherited differently between males and females.
The most well-known example of sex-linked inheritance is the inheritance of hemophilia, a bleeding disorder caused by mutations in the genes that code for blood clotting proteins. Hemophilia is caused by mutations in genes located on the X chromosome, and is therefore more common in males, who have only one X chromosome. Females are typically unaffected by hemophilia because they have a second X chromosome that can provide a working copy of the gene.
Sex-linked inheritance can follow different patterns depending on whether the trait is recessive or dominant. In the case of a recessive sex-linked trait, such as hemophilia, the trait is more likely to be expressed in males because they have only one copy of the X chromosome. Females can be carriers of the trait if they have one copy of the mutated gene, but are typically not affected because they have a second X chromosome that can provide a working copy of the gene.
In the case of a dominant sex-linked trait, such as some forms of intellectual disability, the trait may be expressed in both males and females, but may be more severe in males because they have only one copy of the X chromosome.
Understanding sex-linked inheritance is important for predicting the inheritance of traits and for developing treatments for genetic disorders caused by mutations in sex-linked genes. It can also have implications for genetic counseling and family planning.