Genes that are located on one of the sex chromosomes are said to be sex linked. Sex  linkage results in an inheritance pattern different from that shown/of genes carried on autosomes (chromosomes other than sex chromosomes). Sex linked genes may be on the X-chromosome, hence, they are called X-linkage or on the  Y- chromosomes called Y-linkage.
X-linkage, because the X-chromosome is much larger than the Y-chromosome. Most sex-linked genes are located on the X-chromosomes. In addition to carrying genes that are concerned with the determination of female sex characteristics, the X-chromosome also carries genes for non-sexual characteristics. Such as ability to see particular colours, ability to clot blood efficiently among other traits. In X-linkage, recessive allele is expressed more often in males than in females because the heterogametic sex o.e male carry only one allele for an X-linked gene whereas the homogametic sex (females) have alleles for the gene to be manifested phenotypically.
Thus, a female with one recessive allele on  one of her X-chromosomes will not be affected by it. However, a male with a recessive allele on his X-chromosome has no dominant allele to manifest phenotypically on the presence of a single allele.