Final answer:
Hemophilia is a sex-linked trait that occurs due to a gene on the X chromosome. Males are affected if they inherit an X chromosome containing the hemophilia gene, while females must inherit two such genes to exhibit the disorder.
Step-by-step explanation:
Hemophilia is an example of a sex-linked trait. This is because the gene for hemophilia is located on the X chromosome. Inheritance of sex-linked traits differs from autosomal inheritance because males have only one X chromosome. Therefore, if a male inherits an X chromosome with the hemophilia gene, he will express the disorder since there is no second X chromosome to potentially carry a normal allele. Females, having two X chromosomes, must inherit two copies of the hemophilia gene (one on each X chromosome) to express the condition, making it much rarer in females. Females with one mutated X chromosome and one normal X chromosome are carriers; they typically do not exhibit symptoms but can pass the mutated gene to their offspring.