In humans, males have an X and a Y chromosome while females have two X chromosomes. This means that any gene present on the X chromosome in a males will be expressed even if it is a recessive allele. As females have two X chromosomes they can 'carry' recessive alleles on the X chromosome in the normal way.
There are a number of characteristics which are determined by genes on the X chromosome, and are therefore sex linked. These include colour blindness and haemophilia.
The following example shows a genetic cross involving a carrier female and a normal male. The principles shown in this cross apply to all cases of sex linkage.