Genes which are located on one of the sex chromosomes (X or Y) are said to be sex-linked and their expression will depend on whether the offspring is male (XY) or female (XX). The Y chromosome is much smaller than the X chromosome, so most alleles are carried on the X chromosome (they are X-linked). Men only have one X chromosome which means that they will only inherit one allele for these genes, compared to women who will inherit two. This means that if men inherit a recessive allele (which causes disease) for a gene found on the X-chromosome, they will have the disease. Women who inherit the recessive allele will just be a carrier, since they have another X chromosome with the dominant, functioning allele. For women to have X-linked diseases, they must inherit two disease alleles (they will have a homozygous recessive phenotype). Examples of sex-linked disorders include haemophilia and red-green colour blindness.