Sex linkage is a type of genetic disease, where the gene with the faulty allele is carried on the X chromosome. 

Women have 2X chromosomes whilst men only have 1. Consequently if the diseased allele is recessive, males always express it in their phenotype if it is present in their genotype (X^hY) whereas women require 2 recessive alleles to show expression (homozygous X^hX^h). Therefore women can only be effected if both her mother and father contain the allele. Women can be carriers (X^hX^H). 

A key example is haemophillia.