The blood clotting disease, hemophilia, occurs much more frequently in males because it is caused by a recessive gene carried on the sex chromosomes. As such, it would be referred to as:

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Sex-linked traits are, by definition, those that are determined by genes on the 23rd chromosome pair (the sex chromosomes). Sex-linked traits are not, by definition, heterozygous or homozygous. If a normally recessive allele appears on the male's X chromosome, there often is no allele on the Y chromosome to offset it, and the recessive trait will be expressed as the individual's phenotype. In contrast, in females, the recessive trait will be expressed only if it occurs on both X chromosomes.

The Blood Clotting Disease, Hemophilia, Occurs Much More Frequently in Males