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Sex-linked inheritance is the passing on of traits that are determined by genes on the sex chromosomes, the X and Y chromosomes. Sex-linked traits appear more in males (XY) than females (XX) because the Y chromosome is smaller and holds fewer genes. Females must have two X chromosomes with recessive genes for the sex-linked trait to appear. If there is a recessive allele on the X chromosome, in most cases there is no allele on the Y chromosome that can mask it. If only sons show the trait, that means that the trait is X-linked.